Docker build fails

[andig]

Fails due to missing mbstring extension. Should also check how to upgrade to php 7.3.

[andig]

/cc @tofuSCHNITZEL if this is something you can help with

[tofuSCHNITZEL]

yes I noticed that it was not working for rpi docker image - I want to rework all of it anyway