Support dependent env vars

[marcinc]

Resolves: #391

Compose Environment variables which values are dependent on other environment variables must be ordered properly so that they can be expanded correctly. This PR allows for env variables with templated values that reference other environment variables, e.g.

# docker-compose.yaml

environment:
  MY_VAR: {{FOO}}/{{BAR}}
  FOO: foo
  BAR: bar

In k8s manifest this will translate to:

    spec:
      containers:
        - env:
            - name: FOO
              value: "foo"
            - name: BAR
              value: "bar"
            - name: MY_VAR
              value: "$(FOO)/$(BAR)"

[ezodude]

💯

[ezodude]

This make a lot of sense.

In fact, we need this as a custom method for relying on ENV values, that will pick the value of another ENV value. Esp., that compose doesn't have a native mechanism to do this.

[ezodude]

👍