Smallest docker container with elixir app

In case anyone is interested, this is the container I’m building right now. It uses:

• alpine:3.6 as a builder/runner image,
• Erlang 20.1.1
• Elixir 1.5.2

I like to build my own image instead of a pre-backed image in this case. I like to install elixir/erlang, as well as other packages, instead of compiling them. By looking at base image, I know that there’s no magic going on here.

It’s size, including my apps compiled code and dependencies is 55.5 Mb.

FROM alpine:3.6 as builder

RUN echo "http://dl-cdn.alpinelinux.org/alpine/edge/community/" >> /etc/apk/repositories

RUN apk add --update \
      git \
      erlang=20.1.1-r0 \
      elixir=1.5.2-r0 \
      erlang-crypto \
      erlang-parsetools \
      erlang-syntax-tools \
      erlang-runtime-tools

ADD . /app

WORKDIR /app

ENV MIX_ENV=prod

RUN mix do local.hex --force, \
           local.rebar --force, \
           deps.get, \
           deps.compile, \
           release

FROM alpine:3.6

RUN apk add --no-cache \
      ncurses-libs \
      zlib \
      ca-certificates \
      openssl \
      bash

WORKDIR /app

COPY --from=builder /app/_build/prod/rel/my_app/releases/0.0.1/my_app.tar.gz /app

RUN tar -xzf my_app.tar.gz; rm my_app.tar.gz

I’m relying on distillery for building releases. After the container is built, the app must be started by running bin/my_app foreground in the container:

docker run my_app_image bin/my_app foreground

So, the Dockerfile I’m using has changed significantly. I’m now using two of them:

• a base one, which is updated much less frequently,
• and the apps one, which is ran every time I need to pack the app into a container.

This is how they look like:

Base:

# edenlabllc/elixir:1.5.2

FROM alpine:edge

ENV REFRESHED_AT=2017-11-23

RUN apk add --update \
  erlang=20.1.7-r0 \
  elixir=1.5.2-r0 \
  erlang-crypto \
  erlang-parsetools \
  erlang-syntax-tools \
  erlang-runtime-tools \
  git \
  make

Everyday use:

FROM edenlabllc/elixir:1.5.2 as builder

ARG APP_NAME
ARG APP_VERSION

ADD . /app

WORKDIR /app

ENV MIX_ENV=prod

RUN mix do \
      local.hex --force, \
      local.rebar --force, \
      deps.get, \
      deps.compile, \
      release

FROM alpine:edge

ARG APP_NAME
ARG APP_VERSION

RUN apk add --no-cache \
      ncurses-libs \
      zlib \
      ca-certificates \
      openssl \
      bash

WORKDIR /app

COPY --from=builder /app/_build/prod/rel/${APP_NAME}/releases/${APP_VERSION}/${APP_NAME}.tar.gz /app

RUN tar -xzf ${APP_NAME}.tar.gz; rm ${APP_NAME}.tar.gz

ENV REPLACE_OS_VARS=true \
    APP=${APP_NAME}

CMD ./bin/${APP} foreground

In order to build this, I run:

docker build --tag "my_account/my_app:1.2.3" \
             --file Dockerfile \
             --build-arg APP_VERSION=1.2.3. \
             --build-arg APP_NAME=my_app .

I could merge both Dockerfiles, but it just didn’t make sense to re-install the compile-time dependencies every time I needed to build the app. After all, elixir and erlang release versions much less frequently.

I think openssl package is not needed if Erlang is built with ./configure --disable-dynamic-ssl-lib ....