Support for Docker Compose `build context`

[rtoohil]

Some of my compose files uses build context, i.e.

app:
  build:
    context: ./backend

Which tells compose to look in ./backend for the appropriate files, including Dockerfile. It looks like Build.swift tries to support this, but it hasn't worked in my testing. In my app, there's a Dockerfile in ./backend, but compose up doesn't find it.

Specifying a Dockerfile like:

 app:
    build: 
      context: ./backend
      dockerfile: ./backend/Dockerfile

also fails with this error: Error: /Users/rtoohil/Development/stash is not a child of /Users/rtoohil/Development/stash/./backend