Dockerfile: turn on s390x support

[iii-i]

Hi,

I would like to contribute s390x (IBM Z) support to serval and jitterbug.

The current full branches are here: https://github.com/iii-i/serval/tree/s390 https://github.com/iii-i/jitterbug/tree/s390

The plan is to contribute the pieces in the following sequence:

• Serval Dockerfile (this change)
• Serval support
• Current jitterbug support (w/o verification of certain jumps, prologues and epilogues)
• Finalize jitterbug support at a later point in time

Best regards, Ilya

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• Use Unicorn2.
• Add s390x to UNICORN_ARCH.
• Use a newer Ubuntu base image, since 20.10 has reached EOL and the respective repos are gone.