Add a dotenv function to format configuration for environment files

[lblackstone]

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Issue details

From @pgavlin:

It'd be nice if there was a function like fn::dotenv that took an object and returned a dotenv-formatted string

We can write the following today:

values:
  files:
    DOCKER_ENVFILE: |
      FOO=bar

This would be more flexible as structured configuration along these lines, which would generate an equivalent environment:

values:
  dockerEnv:
    FOO: bar
  files:
    DOCKER_ENVFILE: { fn::dotenv: "${dockerEnv}" }

Affected area/feature