mix docker.release causes a missing environment error in gitlab ci

[imerkle]

#Dockerfile.release 
FROM bitwalker/alpine-elixir:latest

# install gcc make
RUN set -x\
 && apk add --no-cache\
  gcc\
  make\
  musl-dev

EXPOSE 8000
ENV PORT=8000 MIX_ENV=prod REPLACE_OS_VARS=true SHELL=/bin/bash
WORKDIR /app
COPY ./myapp.tar.gz ./
RUN tar xfz myapp.tar.gz
ENTRYPOINT ["bin/myapp"]

$ mix docker.release
** (MatchError) no match of right hand side value: {:error, :missing_environment}
    lib/mix_docker.ex:193: MixDocker.release_version/0
    lib/mix_docker.ex:29: MixDocker.release/1
    (mix) lib/mix/task.ex:301: Mix.Task.run_task/3
    (mix) lib/mix/cli.ex:75: Mix.CLI.run_task/2
    (elixir) lib/code.ex:376: Code.require_file/2

mix docker.build works but not release.
also it works in my own computer just not in ci

[Aleksion]

@dsslimshaddy did you ever find away around this?