Use Alpine Docker images as a base

This reduces resulting image by over 100MB.

.dockerignore

@@ -1,11 +1,13 @@
-/_build
-/deps
-/tests
-/.mc
-/.elixir_ls
-/data
-/cover
 /.DS_Store
+/.elixir_ls
 /.envrc
 /.formatter.exs
+/.mc
+/_build
+/cover
+/data
+/deps
+/erl_crash.dump
 /tags
+/test
+/tmp

Dockerfile

@@ -1,22 +1,31 @@
-FROM ubuntu:latest AS source
-RUN apt-get update && apt-get install -y --no-install-recommends \
-    libssl1.1 imagemagick ghostscript libcap2 sudo \
-    && rm -rf /var/lib/apt/lists/*
+FROM alpine:latest AS source
+RUN apk add --update \
+    bash \
+    ghostscript \
+    imagemagick \
+    libcap \
+    openssl \
+    sudo \
+    && rm -rf /var/cache/apk/*
 ENV LANG C.UTF-8
 ENV PORT 80
 HEALTHCHECK --timeout=5s --interval=10s CMD imager ping
 ENTRYPOINT ["/usr/local/bin/imager"]
 CMD ["foreground"]
 
-FROM appunite/elixir-ci:1.7.1 AS build
-RUN apt-get update && apt-get install libcap-dev
+FROM elixir:1.7.3-alpine AS build
+RUN apk add --update \
+    build-base \
+    git \
+    libcap-dev \
+    && rm -rf /var/cache/apk/*
 ENV MIX_ENV prod
 ENV OPTIMIZE true
 COPY . /app
 WORKDIR /app
-RUN mix local.hex --force && mix local.rebar --force
-RUN mix deps.get
-RUN mix compile && mix release --env=prod
+RUN mix do local.hex --force, local.rebar --force
+RUN mix do deps.get, deps.compile
+RUN mix do compile, release --env=prod
 
 FROM source
 MAINTAINER Łukasz Jan Niemier <lukasz.niemier@appunite.com>