support for s390x is unclear

[iurly]

I see there are images targeting s390x being pushed to dockerhub, but I don't see them listed as officially supported in the main README.md.
I tried using it to build github.com/mozilla/sccache (after disabling LTO which was leading to built-time issues), see mozilla/sccache#2437 but I get a binary which SIGSEV's right from the start.

It appears to me the rust support for s390x has improved during the course of the past couple of years so I was hoping it would be available now. Any way I can contribute to this?

[wojiushixiaobai]

• No rust-std support for s390x-unknown-linux-musl rust-lang/rust#103577
• https://doc.rust-lang.org/nightly/rustc/platform-support/s390x-unknown-linux-musl.html