Multi-stage Dockerfile dev setup

This is an example on how to leverage multi-stage Dockerfile with docker-compose.dev.yml to improve DX with balena.

Principles

• The dockerfile is split in 3 main stages :

  • build (all the preparation steps, including the compilations steps, etc.)
  • prod (imports the important files from build and leave out the scaffolding. Then adds the configurations and commands for production)
  • dev (imports all things needed for test and dev from the build stage. Then adds the configurations and commands for development)

• It's important for dev to be the latest step as we'll want to be able to use balena's live-reload when pushing in local mode and it only works if we run the full Dockerfile (all steps).

• If we were to add a test stage it should go between prod and dev.

• To build a certain stage we use balena build --target _step_ -t _imagename:tag_ . ( i.e. balena build --target prod -t hello:world . )

• We set the target in the docker-compose.yml with target: prod

• Then we set the docker-compose.dev.yml with target: dev so it's overwritten when using local mode push.

i.e. :

services:
  hello:
    build:
      context: .
      target: dev

Limitations

• For live update to work in local mode, the dev step must be the latest step
• docker-compose.dev.yml won't be merged when doing a build (currently only works when pushing in local mode). See this PR for information about how it actually works.