Dockerfile syntax

Documentation

official docs

https://docs.docker.com/engine/reference/builder/

Usage

Images are created from dockerfiles using the build command.

docker build \
  -f /path/to/Dockerfile \
  -t <container_name> \
  /path/for/container

docker build .  # if dockerfile in cwd

# or you can use a directory
# image-name/
#    Dockerfile

docker build  image-name

Syntax

FROM

Every container starts with a from line. This indicates the base image that your container is built on top of.

FROM ubuntu:16.04

RUN

Commands that run when your container is being built for the first time.

RUN apt-get update && \
    apt-get install python3-pip

CMD

commands run every time your container is started.

CMD  echo "welcome!" >> /home/user/welcome.txt

ADD

copy a file into your container.
Supports glob-style matching.

ADD                    /path/file  /path/file
ADD                    Gemfile*    /path/
ADD                    .           /path/
ADD --chmod=user:user  /path/file  /path/file

ENTRYPOINT

If you are creating a container to run a specific application, set that application to the ENTRYPOINT. This means that every time that your container is started, that application will run.

ENTRYPOINT ['/usr/bin/firefox', '-private-window']   ## exec-style (best practice)
ENTRYPOINT /usr/bin/firefox -private-window          ## shell-style (discouraged, unix signals will be unavailable, your program will not be PID #1)

If your entrypoint is a script, you can ensure that it is able to properly handle unix signals by using gosu and exec:

#!/bin/bash
set -e

if [ "$1" = 'postgres' ]; then
    chown -R postgres "$PGDATA"

    if [ -z "$(ls -A "$PGDATA")" ]; then
        gosu postgres initdb
    fi

    exec gosu postgres "$@"
fi

exec "$@"

ENV

Set an environment variable in the container.
Once defined, they can be referred to as $VAR

ENV PYTHONPATH /home/dev/python
ENV blah=$PYTHONPATH

EXPOSE

Exposes a port inside the container to the host. (I am still unclear what interface the port is bound to... 127.0.0.1 ?)

Section Marked as Requiring Revision:

The official documentation says that expose does not actually forward container's ports, it only indicates that the container is listening on these ports. I need to test this to see what is actually happening when this command is used..

EXPOSE 8080        ## expose container's 8080 on host's 8080
EXPOSE 8080 9000   ## expose container's 8080 on host's 9000

ARG

Arg allows you to submit arguments at build time.
It can be accessed later similarly to an environment variable.

ARG VERSION
ARG VERSION=DEFAULT_VALUE
RUN echo $VERSION

docker build --build-arg VERSION=1.1.1

VOLUME

Defines mountpoint for a volume inside the container.
This is optionally bound within docker run -v /src:/dst.
If not defined, then a location will be created automatically.

Volumes can be shared between containers.