Mix deps.get memory explosion when doing cross-platform Docker build

Wow, that’s quite the rabbit hole.

I’m very happy to report that adding ENV ERL_FLAGS="+JPperf true" to the build-section of my Dockerfile fixes this.

Thanks, @pdgonzalez872.

(To anyone else needing this workaround, if you consult the Erlang docs for that flag, this is indeed recommended as a workaround for Qemu-specific problems, not something you should have set on your server).