Allow user to specify a list of plugins to keep when building images

[jefflester]

The new image installer has a hardcoded list of plugins to keep. That list should be extendable through user input – I'm thinking an environment variable that can be sourced inline (e.g. minitrino -e KEEP_PLUGINS="plugin,plugin2" ...), from minitrino.cfg, or from shell environment variables. The value of the variable will be added/merged to the keep list in the image installer.

I'm not sure if this alone will prompt an image rebuild when running minitrino provision. The user might need to run minitrino provision --docker-native --build to force a build, or we could invoke that automatically in the presence of KEEP_PLUGINS.

[jefflester]

@callard-starburst please see: https://github.com/jefflester/minitrino/blob/3.0.0/src/lib/image/src/scripts/download-tarball.py#L161-L175

I'd like to add a test or two for this, and am open to having it be updated in any way that you see fit. We can discuss the details whenever.

[callard-starburst]

@jefflester With the new testing with PyTest, which " test_ " file should we use? I would assume either " cmd_provision " or " cmd_config ".

[jefflester]

@callard-starburst I'd go with the cmd_provision script for this. A heads up that I believe this will require adding --docker native '--build' to the provision command to prompt the build, but that can be tested.

Another thought is we could automatically pass in --build if the env var is detected at runtime.