Build with any environment, defaulting to prod

lib/mix_docker.ex

@@ -16,7 +16,7 @@ defmodule MixDocker do
     Mix.Task.run("release.init", args)
   end
 
-  def build(args) do
+  def build(args \\ []) do
     with_dockerfile @dockerfile_build, fn ->
       docker :build, @dockerfile_build, image(:build), args
     end
@@ -122,7 +122,8 @@ defmodule MixDocker do
   end
 
   defp docker(:build, dockerfile, tag, args) do
-    system! "docker", ["build", "-f", dockerfile, "-t", tag] ++ args ++ ["."]
+
+    system! "docker", ["build", "-f", dockerfile, "-t", tag, "--build-arg",  "MIX_ENV=#{Mix.env}"] ++ args ++ ["."]
   end
 
   defp docker(:create, name, image) do

priv/Dockerfile.build

@@ -1,6 +1,8 @@
 FROM bitwalker/alpine-erlang:19.3
 
 ENV HOME=/opt/app/ TERM=xterm
+ARG MIX_ENV
+ENV MIX_ENV ${MIX_ENV:-prod}
 
 # Install Elixir and basic build dependencies
 RUN \
@@ -17,14 +19,12 @@ RUN mix local.hex --force && \
 
 WORKDIR /opt/app
 
-ENV MIX_ENV=prod
-
 # Cache elixir deps
-RUN mkdir config
+RUN mkdir config       
 COPY config/* config/
 COPY mix.exs mix.lock ./
 RUN mix do deps.get, deps.compile
 
 COPY . .
 
-RUN mix release --env=prod --verbose
+RUN mix release --verbose --env=$MIX_ENV

priv/Dockerfile.release

@@ -3,9 +3,12 @@ FROM bitwalker/alpine-erlang:19.3
 RUN apk update && \
     apk --no-cache --update add libgcc libstdc++ && \
     rm -rf /var/cache/apk/*
+
+ARG MIX_ENV
+ENV MIX_ENV ${MIX_ENV:-prod}
 
 EXPOSE 4000
-ENV PORT=4000 MIX_ENV=prod REPLACE_OS_VARS=true SHELL=/bin/sh
+ENV PORT=4000 REPLACE_OS_VARS=true SHELL=/bin/sh
 
 ADD ${APP}.tar.gz ./
 RUN chown -R default ./releases