It is inconvenient to change the Python dependency set

[exarkun]

On CI, Docker images are built nightly that include the dependencies as declared on master. In a feature branch, if you add to the dependency set, CI jobs run in the same Docker images where the dependencies won't be available. You have to jump through some hoops to get new Docker images built that have your dependencies - and then land your change fast enough so that the Docker images aren't rebuilt from master before you're finished with development.

One way to alleviate the pain here would be to stop automatically rebuilding the images every night and instead have a manual trigger to rebuild them.

This would make the workflow for adding a dependency be:

• Hack on your feature branch to declare the dependency properly
• Trigger the image rebuild
• Continue hacking until you're happy
• Merge to master

However, since there is only one set of Docker images, you also need to coordinate with other folks who might be changing dependencies or rebuilding the images.

[GitHub <noreply@…>]

In 56b93ae/trunk:

Merge pull request #1240 from tahoe-lafs/support-triggered-image-builds

Support triggered image builds

Fixes: ticket:3958