Add musl-dev

`gcc` is already a package installed, but it does not install a `libc` candidate with it. `musl-dev` contains the necessary development files (`crti.o`, `crtn.o`, `Scrt1.o`, `libc.so`, etc.).

This was prior requested in #68 and is necessary for `proc-macro`s/build scripts to compile, without additional configuration/setup. It also is apparently just a 2% size increase which many consumers are already incurring due to how only effectively bare-metal compilation is possible without it.

Author: kayabaNerve

Dockerfile-alpine.template

@@ -4,6 +4,7 @@ LABEL org.opencontainers.image.source=https://github.com/rust-lang/docker-rust
 
 RUN apk add --no-cache \
         ca-certificates \
+        musl-dev \
         gcc
 
 ENV RUSTUP_HOME=/usr/local/rustup \