Refine Docker build and acceptance flow

[ThaddeusJiang]

Summary

• switch the Docker build to a multi-stage flow that still uses an Elixir base image in the final stage
• keep mix, elixir, and erl available in the running container instead of using mix release
• update the acceptance testing skill and compose config to validate the app in the compose network
• add a .dockerignore and record the Docker packaging decision in an ADR

Testing

• docker build -t save_it:test .
• mise run build-acceptance-test
• docker compose up -d cobalt-api typesense typelens
• docker compose --profile prod up --build save_it