Rollup merge of #67102 - Aaron1011:patch-3, r=Mark-Simulacrum

Rollup merge of #67102 - Aaron1011:patch-3, r=Mark-Simulacrum

Add note to src/ci/docker/README.md about multiple docker images

I spent a while debugging a strage linker error about an outdated `glibc` version, only to discover that it was caused by a stale `obj` directory. It wasn't obviously to be that using the same obj dir with multiple Docker images (for the same target triple) could be a problem.

This commit adds a note to the README, which should hopefully be helpful to anyone else who runs into this issue.