CircleCI / Docker configuration to build, test, and push an image to Docker Hub

.circleci_config.yml

# file path should be .circleci/config.yml
version: 2  # use CircleCI 2.0 instead of CircleCI Classic
jobs:  # basic units of work in a run
  build:  # runs not using Workflows must have a `build` job as entry point
    parallelism: 1  # run only one instance of this job
    docker:  # run the steps with Docker
      - image: circleci/elixir:1.11.3  # ...with this image as the primary container; this is where all `steps` will run
        auth:
          username: $DOCKER_USER
          password: $DOCKER_PASS  # context / project UI env-var reference
        environment:  # environment variables for primary container
          MIX_ENV: test

    working_directory: ~/app  # directory where steps will run

    steps:  # commands that comprise the `build` job
      - checkout  # check out source code to working directory

      - run: mix local.hex --force  # install Hex locally (without prompt)
      - run: mix local.rebar --force  # fetch a copy of rebar (without prompt)

      - restore_cache:  # restores saved mix cache
      # Read about caching dependencies: https://circleci.com/docs/2.0/caching/
          keys:  # list of cache keys, in decreasing specificity
            - v1-mix-cache-{{ .Branch }}-{{ checksum "mix.lock" }}
            - v1-mix-cache-{{ .Branch }}
            - v1-mix-cache
      - restore_cache:  # restores saved build cache
          keys:
            - v1-build-cache-{{ .Branch }}
            - v1-build-cache

      - run: |
          mix do deps.get, compile  # get updated dependencies & compile them

          MIX_ENV=dev  mix check # code analysis using `ex_check`
          MIX_ENV=test mix test  # run all tests in project

      - save_cache:  # generate and store mix cache
          key: v1-mix-cache-{{ .Branch }}-{{ checksum "mix.lock" }}
          paths: "deps"
      - save_cache:  # generate and store mix cache
          key: v1-mix-cache-{{ .Branch }}
          paths: "deps"
      - save_cache:  # generate and store mix cache
          key: v1-mix-cache
          paths: "deps"
      - save_cache: # don't forget to save a *build* cache, too
          key: v1-build-cache-{{ .Branch }}
          paths: "_build"
      - save_cache: # don't forget to save a *build* cache, too
          key: v1-build-cache
          paths: "_build"

      - store_test_results:  # upload junit test results for display in Test Summary
          # Read more: https://circleci.com/docs/2.0/collect-test-data/
          path: _build/test/lib/my_app # Replace with the name of your :app

      - setup_remote_docker
      - run:
          name: Build image
          command: |
              docker login -u $DOCKER_USER -p $DOCKER_PASS

              docker build . --build-arg mix_env=prod --build-arg secret_key_base=$SECRET_KEY_BASE -t org/my-app:release
              docker tag org/my-app:release org/my-app:$CIRCLE_SHA1
              docker tag org/my-app:release org/my-app:$CIRCLE_BUILD_NUM

              docker push org/my-app
      - run:
          name: Tag main as latest
          filters:
            branches:
              only: main

          command: |
              docker login -u $DOCKER_USER -p $DOCKER_PASS

              docker tag org/my-app:release org/my-app:latest

              docker push org/my-app

Dockerfile

# ---- Build Stage ----
FROM elixir:1.11.3-alpine AS builder

EXPOSE 80

ENV APP_HOME /app

RUN mkdir $APP_HOME
WORKDIR $APP_HOME

COPY mix.exs .
COPY mix.lock .

ARG mix_env
ENV MIX_ENV $mix_env

ARG secret_key_base
ENV SECRET_KEY_BASE $secret_key_base

RUN mix local.hex --force && \
    mix local.rebar --force && \
    mix deps.get

COPY . /app

RUN MIX_ENV=prod mix release

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

# ---- Application Stage ----
FROM alpine:3
RUN apk add --no-cache --update bash openssl ncurses-libs
WORKDIR /app
COPY --from=builder /app/_build/prod/rel/my_app .
CMD ["/app/bin/my_app", "start"]