Build and test RISCV

[SebastianSchildt]

This builds and pushes RISCV containers as well. One step further towards full #666

I extended the integration test, so that all containers are tested (before only the one similar to the runner architecture)

As the riscv builds need to based on glibc for the moment, there was no easy way to adapt the general docker file, so I switched to building with cross and having the docker just "package" the binaries. On the plus side

• this enables to run on standard Github runners as well
• Does not need the hacky (ingenious?) copy-from-oci action anymore

Once this is in, I'd give the rust-cli the same treatment

[erikbosch]

Looks good to me. I did a very simple smoke test just starting the tll.sh container on amd64. I have not tested the risc part.