In this blog, we'll walk through some core Docker concepts — including containers, volume mounts, persistent data, and building custom Docker images using Dockerfile. If you're getting into DevOps or containerization, this is the hands-on practical you're looking for!
Automatically Clean Up a Docker Container
If you're running short-lived containers (like Alpine Linux), and want to clean them up automatically once they exit, use:
docker run -it --rm alpine
The --rm flag removes the container once it exits, saving disk space and cleanup time.
Mounting the Host File System in a Container
Containers are isolated, so if you create a file in Container1, it’s not available in Container2. But what if you want to share files between the host and your container?
That’s where bind mounts come in.
Why Share Files?
Instead of rebuilding container images every time, you can mount host files/folders into the container using Docker volumes.
Syntax: Bind Mount using -v
docker run -v <host_path>:<container_path> image_name
Example: Mount a Host HTML Directory to Nginx
docker run -v ~/html:/var/www/html nginx
Example: Multiple Volumes
docker run -v ~/pgdata:/opt/data -v pg.conf:/etc/pg.conf postgres
Be sure to use absolute paths. Docker doesn't understand shortcuts like ~.
Task: Passing a File to a Container
Your app crane reads a file named customerdata.json during runtime. Previously, you had to bake that file into every container image. Now, you can pass it at runtime using volume mounts!
Step-by-step:
- Modify the file:
vim ~/workspace/customerdata.json
Update the Company Name to: The File Store
- Run the container:
docker run -v ~/workspace/customerdata.json:/customerdata.json crane
What happens under the hood?
When you mount a file to /customerdata.json, it overrides the internal default file in the image.
The container still works if you don’t pass the file — it just falls back to the default.
Persistent Volumes
Bind mounts are one-time mounts, but Docker also supports persistent named volumes using docker volume or Docker Compose. These help preserve data across container restarts or recreations.
Building a Custom Jenkins Image with Dockerfile
Let’s build a custom Docker image for Jenkins using a Dockerfile.
- Dockerfile (Example):
FROM openjdk:11-jdk
LABEL maintainer="Ankit"
LABEL env=production
ENV apparea=/home/ankit/Practice
RUN mkdir -p $apparea
ADD https://get.jenkins.io/war/2.397/jenkins.war $apparea
WORKDIR $apparea
EXPOSE 8080
CMD ["java", "-jar", "jenkins.war"]
- Build the Docker Image:
sudo docker build -t jenkins-custom .
- Run the Jenkins Container:
sudo docker run -d -p 8080:8080 jenkins-custom
- Check Running Containers:
sudo docker ps -a
Docker vs Dockerfile
| Feature | Dockerfile | Docker Compose |
|---|---|---|
| Focus | Build a single image | Run multiple containers |
| File Name | Dockerfile (no extension) | docker-compose.yml |
| Commands | FROM, RUN, CMD, COPY, etc. | services, volumes,etc |
| Use Case | Single-container builds | Multi-container applications |
What is docker build?
The docker build command creates Docker images from a Dockerfile.
Syntax:
docker build -t <image-name> .
-t lets you tag the image.
. is the build context (typically the current directory).
Common Dockerfile Instructions
| Instruction | Purpose |
|---|---|
| FROM | Base image (e.g., Ubuntu, Alpine) |
| RUN | Execute commands (e.g., install tools) |
| COPY | Copy files from host to image |
| ADD | Like COPY but supports remote URLs |
| CMD | Default command for container |
| ENTRYPOINT | Similar to CMD but allows arguments |
| EXPOSE | Document container ports |
Final Thoughts
With just a few Docker commands and a solid Dockerfile, you can:
Mount files from your host system
Avoid rebuilding images repeatedly
Build production-ready images like Jenkins
Simplify container deployment
Have questions or want a full docker-compose.yml example in the next post? Drop a comment or follow me!
Thanks for reading! 🙌
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