Add option for propagating all environment variables

[zsims]

Per the Elastic CI for AWS guidance (https://github.com/buildkite/elastic-ci-stack-for-aws#what-type-of-builds-does-this-support):

Each project should encapsulate it's dependencies via Docker and Docker Compose

This results in using this plugin for setting up build dependencies. It would be nice to add an option to propagate all environment variables to the container, e.g.

steps:
  - command: "echo \$BUILDKITE_COMMIT"
    plugins:
      - docker#v2.0.0:
         image: bash 

Rather than having to specify that BUILDKITE_COMMIT propagates to the container (with environment)

[lox]

Agreed! Any suggestions on what the syntax for that would look like?

steps:
  - command: "echo \$BUILDKITE_COMMIT"
    plugins:
      - docker#v2.0.0:
         image: bash
         build-environment: true

[zsims]

I think something like propagate-env: true or include-all-env: true or automatic-environment: true avoiding something like build as that is a bit confusing re docker build

steps:
  - command: "echo \$BUILDKITE_COMMIT"
    plugins:
      - docker#v2.0.0:
         image: bash
         propagate-env: true

[toolmantim]

Just to be clear… we'd just want the job environment variables sent down from the web UI yeah (the ones in the environment tab)? Not necessarily any environment variables picked up from the Elastic CI Stack environment? (like secrets)

[zsims]

I would have thought all of them, e.g what you'd normally have in a naked step?

[lox]

Yeah, I'd imagine that we'd load all of them via the envfile.

[zsims]

@toolmantim / @lox have you had any time to poke #80?