Freeze versions of container used for cross-compilation to keep compatibility with older GLIBC

[pietrygamat]

Keep building on the same stack we did 2.9.6 is the simplest fix to #190

[tresf]

Thanks!

• Since the commits are on your fork, I think this prevents us from manually running the workflow to create the binaries.
• Locking the system into an older dockcross ref: I assume this would prevent us from using improvements or updates from upstream? So as a long-term solution, I believe we should ideally (eventually) use an upstream-maintained container specifically intended for compatibility with older glibc versions (if such a thing exists, if not we may want to PR this against dockcross). This seems like a simple stop-gap for the interm however.
• Furthermore -- I think I understand the desired simplicity of this approach -- but I'm uncertain if applying this to Windows builds has any benefit? Windows binaries shouldn't normally link against glibc and would generally be out of scope for Is glibc 2.35 required now? #190 if my memory serves correctly.