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David Tio
David Tio

Posted on • Originally published at blog.dtio.app

Docker Environment Management: Images, Logs, and Cleanup (2026 Guide)

#devops #docker #tutorial #linux

Docker Environment Management: Images, Logs, and Cleanup (2026 Guide)

Quick one-liner: Learn how to manage your Docker environment like a pro — list and remove images, view container logs, use environment variables, and reclaim disk space.

Why This Matters

After running a few containers, your Docker host starts accumulating stuff — images, stopped containers, unused volumes, build caches.

I learned this the hard way. A few months into using Docker, I ran df -h and discovered my home directory was 90% full. Docker had quietly consumed gigabytes of images, orphaned volumes, and build layers.

That's when I learned the prune commands.

This guide covers the essential skills for managing your Docker environment:

  • Listing and removing images
  • Viewing container logs
  • Using environment variables (critical for databases)
  • Setting restart policies
  • Cleaning up disk space safely

Prerequisites

  • Docker installed (rootless mode recommended)
  • Basic Docker familiarity (run, stop, rm commands)
  • 5 minutes to work through the examples

Managing Images

Every time you run a container from an image you haven't used before, Docker downloads it and stores it locally. Over time, these images consume significant disk space.

List All Images 

$ docker images
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You'll see something like:

IMAGE                      ID             DISK USAGE   CONTENT SIZE
nginx:latest               dec7a90bd097        240MB         65.8MB
redis:latest               315270d16608        204MB         55.3MB
ghcr.io/jqlang/jq:latest   4f34c6d23f4b       3.33MB         1.03MB
hello-world:latest         85404b3c5395       25.9kB         9.52kB
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The SIZE column shows the compressed size on disk. Just four images from following this series and you're already at ~450MB. Multiply that across months of pulling images and you can see why knowing how to manage them matters.

Remove an Image

To remove an image you no longer need:

$ docker rmi hello-world:latest
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Note: If a container is still using the image (even a stopped one), Docker will refuse to remove it. Stop and remove the container first, or use the -f flag to force removal.

To remove all unused images at once, see the Cleaning Up Disk Space section below.

Viewing Container Logs

When a container fails to start or behaves unexpectedly, the first thing to check is its logs. Docker captures everything a container writes to standard output and standard error.

View Logs 

$ docker logs dtnginx
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You should see the nginx startup messages:

/docker-entrypoint.sh: /docker-entrypoint.d/ is not empty, will attempt to perform configuration
/docker-entrypoint.sh: Looking for shell scripts in /docker-entrypoint.d/
/docker-entrypoint.sh: Launching /docker-entrypoint.d/10-listen-on-ipv6-by-default.sh
10-listen-on-ipv6-by-default.sh: info: Getting the checksum of /etc/nginx/conf.d/default.conf
10-listen-on-ipv6-by-default.sh: info: Enabled listen on IPv6 in /etc/nginx/conf.d/default.conf
/docker-entrypoint.sh: Sourcing /docker-entrypoint.d/15-local-resolvers.envsh
/docker-entrypoint.sh: Launching /docker-entrypoint.d/20-envsubst-on-templates.sh
/docker-entrypoint.sh: Launching /docker-entrypoint.d/30-tune-worker-processes.sh
/docker-entrypoint.sh: Configuration complete; ready for start up
2026/03/23 10:08:54 [notice] 1#1: using the "epoll" event method
2026/03/23 10:08:54 [notice] 1#1: nginx/1.29.6
2026/03/23 10:08:54 [notice] 1#1: built by gcc 14.2.0 (Debian 14.2.0-19)
2026/03/23 10:08:54 [notice] 1#1: OS: Linux 6.8.0-100-generic
2026/03/23 10:08:54 [notice] 1#1: getrlimit(RLIMIT_NOFILE): 1048576:1048576
2026/03/23 10:08:54 [notice] 1#1: start worker processes
2026/03/23 10:08:54 [notice] 1#1: start worker process 29
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The key line to look for is Configuration complete; ready for start up — that confirms nginx started successfully. Everything after that is the worker process startup.

Follow Logs in Real Time

To watch logs as they're written (like tail -f):

$ docker logs -f dtnginx
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Press Ctrl+C to stop following.

Show Last N Lines

To see only the last 10 lines:

$ docker logs --tail 10 dtnginx
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Combine flags for real-time tailing:

$ docker logs -f --tail 20 dtnginx
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Environment Variables

Many Docker images are configured through environment variables rather than configuration files. This makes containers flexible — the same image can behave differently depending on the variables you pass at runtime.

Setting Environment Variables

Use the -e flag to set environment variables when running a container:

$ docker container run -d --name dtpostgres \
    -e POSTGRES_PASSWORD=docker \
    -e POSTGRES_DB=testdb \
    postgres
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This starts PostgreSQL with:

  • Root password set to docker
  • A database called testdb automatically created

Important: Without POSTGRES_PASSWORD, the PostgreSQL container will refuse to start. It's a security requirement.

Verify Environment Variables

You can verify the variables inside a running container:

$ docker exec dtpostgres env
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You should see:

POSTGRES_PASSWORD=docker
POSTGRES_DB=testdb
...
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Connect and Use the Database

Once the container is running, connect using the PostgreSQL client inside the container:

$ docker exec -it dtpostgres psql -U postgres -d testdb
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You're now inside the PostgreSQL shell. Create a table and insert some data:

CREATE TABLE users (
    id   SERIAL PRIMARY KEY,
    name VARCHAR(50)
);

INSERT INTO users (name) VALUES ('Alice');
INSERT INTO users (name) VALUES ('Bob');

SELECT * FROM users;
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You should see:

 id | name
----+-------
  1 | Alice
  2 | Bob
(2 rows)
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Exit the PostgreSQL shell:

\q
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This confirms that environment variables aren't just startup flags — they configure a working database that you can immediately connect to and use.

Common Environment Variables

Image Variable Purpose
postgres POSTGRES_PASSWORD Required — database password
postgres POSTGRES_DB Optional — database to create on startup
postgres POSTGRES_USER Optional — custom username (default: postgres)
mysql MYSQL_ROOT_PASSWORD Required — root password
redis — No env vars required
nginx — No env vars required

Pro tip: Each image documents its supported environment variables on its Docker Hub page. Always check before running.

Cleanup 

$ docker stop dtpostgres
$ docker rm dtpostgres
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Restart Policies

By default, a container stays stopped if it crashes or if the Docker host reboots. Restart policies tell Docker to automatically restart containers.

Available Restart Policies

Policy Behavior
no Do not restart (default)
on-failure Restart only if the container exits with a non-zero exit code
always Always restart, including after host reboot
unless-stopped Like always, but does not restart if you manually stopped the container

Run Container with Restart Policy 

$ docker run -d --restart unless-stopped --name dtnginx nginx
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If you reboot your Docker host, this container will start automatically. If you manually run docker stop dtnginx, it will stay stopped until you explicitly start it again.

Update Restart Policy of Existing Container 

$ docker update --restart unless-stopped dtnginx
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Cleanup 

$ docker stop dtnginx
$ docker rm dtnginx
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Cleaning Up Disk Space

After working with Docker for a while, your host will have accumulated:

  • Stopped containers
  • Unused images
  • Dangling build layers
  • Orphaned volumes

Docker provides prune commands to reclaim this space.

Check Disk Usage

See how much disk space Docker is using:

$ docker system df
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You'll see something like:

TYPE            TOTAL     ACTIVE    SIZE      RECLAIMABLE
Images          5         2         1.2GB     800MB (66%)
Containers      3         1         50MB      40MB (80%)
Local Volumes   2         1         200MB     100MB (50%)
Build Cache     10        0         300MB     300MB (100%)
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The RECLAIMABLE column shows how much space you can free up.

Remove Stopped Containers 

$ docker container prune
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Remove Unused Images 

$ docker image prune -a
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Clean Up Everything at Once

To clean up stopped containers, unused images, networks, and build cache:

$ docker system prune -a
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Docker will ask for confirmation before proceeding. This is a safe way to reclaim disk space, but make sure you don't need any of the stopped containers or unused images before running it.

Exercise

This exercise demonstrates something important — and sets up exactly why you'll need Docker volumes in the next post.

Your Tasks

  1. Start a MySQL container named dtmysql with MYSQL_ROOT_PASSWORD=docker and MYSQL_DATABASE=testdb
  2. Verify the environment variables are set inside the running container
  3. Connect to MySQL and create a users table, insert a row with the name Alice, then query it back
  4. Stop and delete the container
  5. Start a fresh dtmysql container with the same environment variables and try to query the users table again
  6. Clean up

Try it yourself before reading the solution below.

Solution

1. Start MySQL:

$ docker run -d --name dtmysql \
    -e MYSQL_ROOT_PASSWORD=docker \
    -e MYSQL_DATABASE=testdb \
    mysql
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2. Verify env vars:

$ docker exec dtmysql env | grep MYSQL
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3. Connect and insert data:

$ docker exec -it dtmysql mysql -u root -pdocker testdb
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Inside the MySQL shell:

CREATE TABLE users (name VARCHAR(50));
INSERT INTO users VALUES ('Alice');
SELECT * FROM users;
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You should see:

+-------+
| name  |
+-------+
| Alice |
+-------+
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exit
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4. Stop and delete the container:

$ docker stop dtmysql
$ docker rm dtmysql
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5. Start a fresh container and query:

$ docker run -d --name dtmysql \
    -e MYSQL_ROOT_PASSWORD=docker \
    -e MYSQL_DATABASE=testdb \
    mysql
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Wait a few seconds for MySQL to initialise, then:

$ docker exec -it dtmysql mysql -u root -pdocker testdb -e "SELECT * FROM users;"
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ERROR 1146 (42S02): Table 'testdb.users' doesn't exist
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Alice is gone. The container was deleted, and everything inside it went with it.

This is the fundamental problem with containers: they're ephemeral by design. For stateless apps like nginx, that's fine. For databases, it's a disaster.

Coming up next: Docker volumes — the solution to exactly this problem.

6. Clean up:

$ docker stop dtmysql && docker rm dtmysql
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What's Next

Now you can manage your Docker environment effectively:

  • Keep images organized — remove what you don't need
  • Debug with logs — find issues quickly
  • Configure with env vars — run databases and other configurable images
  • Auto-restart containers — survive reboots
  • Reclaim disk space — prune safely

Coming up: Docker persistent volumes — keep your data safe across container restarts and removals.

Want More?

This guide covers the basics from Chapter 4: Managing Docker Environment in my book, "Levelling Up with Docker" — 14 chapters of practical, hands-on Docker guides.

📚 Grab the book: "Levelling Up with Docker" on Amazon

Found this helpful?

Published: 23 Mar 2026
Author: David Tio
Word Count: ~1,100

SEO Metadata:

  • Title: Docker Environment Management: Images, Logs, and Cleanup (2026 Guide)
  • Meta Description: Learn how to manage your Docker environment — list and remove images, view container logs, use environment variables, and reclaim disk space. Includes a hands-on MySQL exercise that shows exactly why volumes matter.
  • Target Keywords: docker images command, docker logs, docker environment variables, docker system prune, docker image prune, docker cleanup, docker env flag, beginner docker tutorial 2026

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