I apologize, I wasn't sure which component the container builds fall under.
When trying to use the container for midPoint 4.2, the external JDBC drivers I'm putting into $midpoint_home/lib are not being found.
I tested it by using the demo/simple docker-compose stack from the container source. First, I explicitly set the FROM of the midpoint_server/Dockerfile explicitly to evolveum/midpoint:4.1 and started up the stack. I was able to add external jars to the lib directory and add a databaseTable resource that utilized the jar.
When I shut the stack down and rebuilt it with the evolveum/midpoint:4.2 source image and went through the same steps, I started to get an error that the driver couldn't be found when I tried to add the resource.
It appears that the classpath isn't getting set correctly, so the contents of the lib directory are not being found.