By the way, since docker released rootless support, I do not use podman, because it is still buggy and podman-compose is still in a buggy alpha state

Of course. Here is my docker-compose file:

version: "3"
services:
  app:
    image: localhost/phoenix_app_${INSTANCE_NAME}:${phoenix_app_INSTANCE_VERSION}
    # image: docker.io/hexpm/elixir:1.14.0-erlang-25.1-alpine-3.16.2
    ports:
      - "8083:8083"
    depends_on:
      db:
        condition: service_healthy
    environment:
      - HOSTNAME=localhost
      - DATABASE_URL=ecto://user:pass@db/phoenix_app
      - SECRET_KEY_BASE=${SECRET_KEY_BASE}
      - PORT=8083
    networks:
      mynetwork:
        aliases:
          - api.local
    # command: ["eval", "IO.inspect :inet_res.resolve('db', :in, :a)"]
    # command: ["eval", "IO.inspect :gen_tcp.connect('db', 5432, [packet: :raw, mode: :binary, active: false], 3000)"]

  db:
    image: postgres:13-alpine
    environment:
        - POSTGRES_USER=user
        - POSTGRES_PASSWORD=pass
        - POSTGRES_DB=phoenix_app
    networks:
      mynetwork:
        aliases:
          - db.local
    healthcheck:
      test: [ "CMD", "pg_isready", "-q", "-d", "phoenix_app", "-U", "user" ]
      timeout: 45s
      interval: 10s
      retries: 10

  server:
    image: caddy:2-alpine
    ports:
        - ${PORT}:80
        - ${SSL_PORT}:443
    volumes:
        - caddy_data:/data
        - caddy_config:/config
        # - ./Caddyfile:/etc/caddy/Caddyfile
volumes:
    caddy_data:
    caddy_config:

networks:
  mynetwork:
    driver: bridge

And my Containerfile if that is at all interesting:

ARG ELIXIR_VERSION=1.14.0
ARG OTP_VERSION=25.1
ARG ALPINE_VERSION=3.16.2

ARG BUILDER_IMAGE="hexpm/elixir:${ELIXIR_VERSION}-erlang-${OTP_VERSION}-alpine-${ALPINE_VERSION}"
ARG RUNNER_IMAGE="alpine:${ALPINE_VERSION}"

FROM ${BUILDER_IMAGE} as build

ARG MIX_ENV="prod"

# install build dependencies
RUN apk add --no-cache build-base git python3 curl

# prepare build dir
WORKDIR /app

# install hex + rebar
RUN mix local.hex --force && \
    mix local.rebar --force

# set build ENV
ARG MIX_ENV
ENV MIX_ENV="${MIX_ENV}"

ARG FORCE_SSL
ENV FORCE_SSL="${FORCE_SSL}"

# install mix dependencies
COPY mix.exs mix.lock ./
RUN mix deps.get --only $MIX_ENV
RUN mkdir config

# copy compile-time config files before we compile dependencies
# to ensure any relevant config change will trigger the dependencies
# to be re-compiled.
COPY config/config.exs config/$MIX_ENV.exs config/
RUN mix deps.compile

COPY priv priv

# note: if your project uses a tool like https://purgecss.com/,
# which customizes asset compilation based on what it finds in
# your Elixir templates, you will need to move the asset compilation
# step down so that `lib` is available.
COPY assets assets
RUN mix assets.deploy

# compile and build the release
COPY lib lib
RUN mix compile
# changes to config/runtime.exs don't require recompiling the code
COPY config/runtime.exs config/
# uncomment COPY if rel/ exists
# COPY rel rel
RUN mix release

# start a new build stage so that the final image will only contain
# the compiled release and other runtime necessities
FROM ${RUNNER_IMAGE} AS app
RUN apk add --no-cache libstdc++ libgcc openssl ncurses-libs ffmpeg

ARG USER
ENV USER="userman"

# Podman bug where MIX_ENV doesn't defined
ARG MIX_ENV="prod"
ENV MIX_ENV="${MIX_ENV}"

WORKDIR "/home/${USER}/app"
# Creates an unprivileged user to be used exclusively to run the Phoenix app
RUN \
  addgroup \
   -g 1000 \
   -S "${USER}" \
  && adduser \
   -s /bin/sh \
   -u 1000 \
   -G "${USER}" \
   -h "/home/${USER}" \
   -D "${USER}" \
  && su "${USER}"

# Everything from this line onwards will run in the context of the unprivileged user.
USER "${USER}"

COPY --from=build --chown="${USER}":"${USER}" /app/_build/"${MIX_ENV}"/rel/phoenix_app ./

ENTRYPOINT ["bin/phoenix_app"]

# Usage:
#  * build: sudo docker image build -t elixir/my_app .
#  * shell: sudo docker container run --rm -it --entrypoint "" -p 127.0.0.1:4000:4000 elixir/my_app sh
#  * run:   sudo docker container run --rm -it -p 127.0.0.1:4000:4000 --name my_app elixir/my_app
#  * exec:  sudo docker container exec -it my_app sh
#  * logs:  sudo docker container logs --follow --tail 100 my_app
CMD ["start"]

I’m really very stumped with this one, so any suggestions are appreciated!