Using Docker for your Elixir Phoenix application

A nifty way to setup an Elixir development environment is to use Docker instead of using asdf or installing the Elixir toolchain locally.

Docker has the option to mount a directory into the container which we can exploit to mount whatever folder we are currently in by using the unix command pwd (print working directory).

We can also instruct Docker to expose a port and remove the container once it is stopped. Putting these things together we can get a Elixir environment entirely contained in Docker. The base of the command would be this in bash:

docker run --mount type=bind,source=$(pwd),target=/app -p 4000:4000 --rm elixir:latest

From this point we can extend it to do whatever we want with Elixir, such as running mix commands.
Although we have to create a Docker image that has these tools contained. Here is an example of a Docker image that you can build with docker build -t elixir-env . in the directory of the Dockerfile:

# Extend from the official Elixir image
FROM elixir:latest

RUN mix local.hex --force \
    && mix archive.install --force hex phx_new \
    && apt-get update \
    && curl -sL https://deb.nodesource.com/setup_lts.x | bash \
    && apt-get install -y apt-utils \
    && apt-get install -y nodejs \
    && apt-get install -y build-essential \
    && apt-get install -y inotify-tools \
    && mix local.rebar --force

ENV APP_HOME /app
RUN mkdir -p $APP_HOME
WORKDIR $APP_HOME

EXPOSE 4000

CMD ["mix", "phx.server"]

After creating the image and tagging it with elixir-env we can create a Phoenix application by running that container in bash:

docker run --mount type=bind,source=$(pwd),target=/app -p 4000:4000 --rm elixir-env:latest mix phx.server

But where this becomes really interesting is when you alias it to a function in your shell.
Doing that will give you the base CLI tool and make it just as accessible as having it installed locally.
To alias the function you simply do:

alias mix="docker run --mount type=bind,source=$(pwd),target=/app -p 4000:4000 --rm elixir:latest mix"

From here we can simply use mix to run any further invocations of that container. Thus we can replace it with the local installation and be oblivious to it running through Docker.

This technique is quite powerful for testing out environments or if you can't get a specific program to run locally on your machine because of conflicting dependencies. Creating a program running in Docker saves you from dealing with global dependencies and allows you the flexibility of a clean installation.