Dockerfile Teardown

So your app is ready, and now you want to containerize it using docker.If yes,then read on.

Docker file is a file which contains all commands that are required to generate an image.

Simple Docker file

Creating a Docker file.

In your project directory create a new file with the exact name Dockerfile.

Inside this file only we will have all the commands listed.Lets see what are those commands.

1. FROM

FROM <image>:<tag>

Dockerfile must start with FROM. This basically sets the base image.

# Example

FROM node:13-alpine

Instructed to get the base image of node with version 13.

2. LABEL

LABEL <key><value> <key2><value2>

Used to set metadata for the image.It is a key-value pair

# Example

LABEL companyname="xyz organization"

Sets companyname label.You can access it using docker inspect

3. RUN

RUN <command>

Executes command inside the new image and commits the results.

# Example

RUN mkdir -p /home/app

This will create a new directory inside the image

3. CMD

CMD [<command>,<command>]

The first commands to be executed after container is created.

# Example

CMD ["npm", "start"]

This will execute npm start when container is created.

4. EXPOSE

EXPOSE <port>

Informs Docker that the container listens on the specified network ports.This information is used to interconnect containers using links and to set up port redirection on the host system

# Example

EXPOSE 3000

This will expose port inside the container as well as for host machine.

5. ENV

ENV <key><value>

Sets environment variables to the value .This is passed to all RUN , CMD instructions

# Example

ENV MONGO_DB_USERNAME=admin

Here MONGO_DB_USERNAME env is set to admin

6. COPY

COPY <source> <destination>

Copies files from source i.e the host into image.All new files & directories are created with mode 0755.

# Example

COPY ./package.json /home/app

Here package.json file is being copied from current directory to /home/app in image.

7. ADD

ADD <source> <destination>

Same as copy but some slight change.

• The ADD instruction can copy and extract TAR files from the Docker host to the Docker image.

# Example

ADD unzipthis.tar /home/app

This will extract files inside the image to the specified directory.

8. VOLUME

VOLUME <path> 

Used for persistence inside docker images.This instruction can create a directory inside the docker image, which can later be mounted to a directory

# Example

VOLUME /data 

9. WORKDIR

WORKDIR <path> 

Instruction specifies what the working directory should be inside the Docker image

# Example

WORKDIR /home/app

This will the directory where the RUN or other instruction will be executed.

Now try Reading this dockerfile again

Hope this will make writing Dockerfile easier for you.