Improve nightly build to build all PRs and build containers

[rfay]

What happened (or feature request):

In #722 we removed the git submodules that were used for a full nightly build, so now although we have a nightly build it does nothing more than a regular build.

As a followup task we want to improve the nightly build to:

• Build all open PRs
• Build the containers and test with them.

What you expected to happen:

How to reproduce this:

Version: Please include the output of ddev version, docker version and the project's .ddev/config.yaml.

Anything else do we need to know:

Related source links or issues:

Please use a complexity rating of 1-5 (5 is high) for a feature request. (High complexity implies more PR planning)

[rickmanelius]

I'm assuming this is a relatively high priority to ensure our test coverage is meaningful.

[rfay]

Nice to have, but not sure it has to be super-high priority.