Support setting environment variables directly from config.yaml

[rfay]

Is your feature request related to a problem? Please describe.

TYPO3 uses environment variables (in php) for various uses that are important to development, especially TYPO3_CONTEXT=Development.

We have various ways to set these but they're pretty twiddly. See https://stackoverflow.com/questions/50109020/how-can-i-set-environment-variables-inside-ddevs-containers and https://stackoverflow.com/questions/49997842/how-to-set-typo3-context-in-ddev

Describe the solution you'd like

Add environment variable spec to config.yaml:

environment:
  web:
    - WEBVAR1=value1
    - WEBVAR2=value2:
  db:
    - DBVAR1=value1

Related PR that does this in a more complicated way: #870

Note: After this is addressed the related Stack Overflow articles should be addressed.

[rfay]

I think the best way to do this is with Stop use of environment variables to communicate with docker-compose. Instead of templating, we'll use a structure that gets marshalled as the docker-compose.yaml. This is not too difficult, but requires changes in a number of places; all data will be included directly in the docker-compose. It will be a really good refactoring I think.