s390x: Introduce s390x CI container

[fneddy]

Summary of the PR

I would like to use seccompiler on s390x.

To get the CI running a new dev image is needed just like for riscv.
I copied the riscv setup and striped it down for s390x. This are my changes:

• use vanilla kernel instead of custom build
• use vanilla qemu instead of custom build
• use initrd to get 9p boot running.
• no riscv specific drivers

Conceptually its the same as the riscv image:

• create a s390x dev image (rootfs_builder)
• install all dev dependencies
• create a native dev image
• copy (rootfs_builder) into the native image at /opt/rootfs
• setup auto boot of the newly created rootfs via qemu-system-s390x in container entrypoint

Requirements

Before submitting your PR, please make sure you addressed the following
requirements:

• All commits in this PR have Signed-Off-By trailers (with
  git commit -s), and the commit message has max 60 characters for the
  summary and max 75 characters for each description line.
• All added/changed functionality has a corresponding unit/integration
  test.
• All added/changed public-facing functionality has entries in the "Upcoming
  Release" section of CHANGELOG.md (if no such section exists, please create one).
• Any newly added unsafe code is properly documented.

[fneddy]

Note:

• the push-to-registry flag is not set yet
• as rust-vmm itself is by far not ready to compile on s390x its not mentioned in the README.md (yet). My aim at this time is to get big-endian and s390x support into seccompiler.