Introduce polkadot-musl-builder

[joao-paulo-parity]

Note that I will not able to follow up on the reviews for this PR because I'm going on vacation. I put this together before leaving in hopes it'll be easier for someone else to take over this work.

Additionally https://github.com/joao-paulo-parity/polkadot/blob/musl/Dockerfile is the link of where the script was ported from in case something got lost in translation.

Note: As documented in the script, I'm defaulting POLKADOT_BRANCH to v0.9.9-1 because lots of jemalloc undefined symbol errors appeared after the switch tikv-jemallocator fork (https://github.com/paritytech/polkadot/pull/3897). Good news is that it's a single error which only happens during the very last step, when it's assembling the binary, and apparently does not seem to be difficult to solve - it might require an upstream PR on https://github.com/tikv/jemallocator though.

related to https://github.com/paritytech/release-engineering/issues/193

[rcny]

Even that it's stale and really outdated, IMO this contribution is important, I'll keep it open to be picked up by someone maybe sometime in the future.

cc @paritytech/release-engineering