WIP: leaner flutterbuilder container

[GeertJohan]

I came across this while using the hover container, needs proper testing though, so PR in WIP.

[provokateurin]

I think we can also update the ubuntu version to focal for all stages, so there are less layers involved and the image should be smaller.

[GeertJohan]

I've merged the MR to update the ubuntu version.

[provokateurin]

Maybe we should also add it to the other apt-get install commands, but if I know it correctly it won't affect the image size, but only the build time.