devops-novice

Dockerfile

A Dockerfile is a text document without an extension which contains all the commands in order which Docker daemon can use to create a docker image.

Dockerfile > build > Docker Image > run > Docker Container

A Dockerfile is used to create a docker image, which is then used to create a docker container.

A Dockerfile can be thought of as a class, docker image as an object and docker container as a process.

Docker	Java
Dockerfile	Class
Docker Image	Object
Container	Process

This analogy makes it simple to understand.

A Dockerfile must be saved as a file Dockerfile

Let us see a sample Dockerfile

FROM ubuntu:18.04
RUN apt update
RUN apt install -y nginx
ADD run.sh /
ENTRYPOINT ["bash","run.sh"]

Save above file with name Dockerfile.

FROM creates layer from ubuntu:18.04 Docker Image
RUN will execute command apt update and apt install -y nginx This will create layer of nginx installation. This container will have nginx installed
ADD will add run.sh in image
ENTRYPOINT will specify action to perform on container start

Each line in Dockerfile will add a new writable container layer to Docker Image.

docker build

The docker build command is used to create image from Dockerfile and context.

The context is set of files at a specified location PATH

Dockerfile and all other needed files/directories needed to create docker image should be present at PATH

During the build, the build process sends entire context(recursively) to the docker daemon. So it is recommended to have only required files in the context to create an image.

Build command to use current directory as context

docker build .

By default docker image will be created with IMAGE_NAME:TAG as <none>:<none> with unique Image ID.

You can specify your IMAGE_NAME:TAG using -t with docker build

docker build -t IMAGE_NAME:TAG .

Build command to specify different location of Dockerfile

docker build -t IMAGE_NAME:TAG -f /path/to/Dockerfile .

Build from git repo

docker build -t IMAGE_NAME:TAG https://github.com/pathto/repo.git

The repo.git should contain Dockerfile