Build fails on latest Crystal nightly image

[Blacksmoke16]

docker run --rm crystallang/crystal:nightly-alpine crystal eval 'puts "foo"'

Produces:

/usr/lib/gcc/x86_64-alpine-linux-musl/9.3.0/../../../../x86_64-alpine-linux-musl/bin/ld: /usr/bin/../lib/crystal/libgc.a(pthread_support.o): in function `GC_thr_init':
pthread_support.c:(.text+0xe39): undefined reference to `gnu_get_libc_version'
/usr/lib/gcc/x86_64-alpine-linux-musl/9.3.0/../../../../x86_64-alpine-linux-musl/bin/ld: /usr/bin/../lib/crystal/libgc.a(mach_dep.o): in function `GC_with_callee_saves_pushed':
mach_dep.c:(.text+0x3b): undefined reference to `getcontext'
/usr/lib/gcc/x86_64-alpine-linux-musl/9.3.0/../../../../x86_64-alpine-linux-musl/bin/ld: /usr/bin/../lib/crystal/libgc.a(pthread_start.o): in function `GC_inner_start_routine':
pthread_start.c:(.text+0x44): undefined reference to `__pthread_register_cancel'
/usr/lib/gcc/x86_64-alpine-linux-musl/9.3.0/../../../../x86_64-alpine-linux-musl/bin/ld: pthread_start.c:(.text+0x6b): undefined reference to `__pthread_unregister_cancel'
collect2: error: ld returned 1 exit status
Error: execution of command failed with code: 1: `cc "${@}" -o /root/.cache/crystal/crystal-run-eval.tmp  -rdynamic -L/usr/bin/../lib/crystal -lpcre -lm -lgc -lpthread -levent -lrt`

First noticed via https://github.com/Blacksmoke16/oq/runs/3875659755?check_suite_focus=true.

[straight-shoota]

Ah, that's caused by the new folder structure of make install. The new lib directory is not excluded in the dockerfile and thus the libgc from the tarball gets into the image. It has a higher precedence than the one specifically built for musl libc. Removing the libgc.a mentioned in the error should be sufficient to fix this.