Added a bunch of packets to musl Dockerfile, inspired from the rust APKBUILD

Author: strfry

ci/docker/x86_64-unknown-linux-musl/Dockerfile

@@ -7,6 +7,21 @@ RUN apk update && \
   perl \
   make \
   gcc \
-  bash
+  bash \
+  musl-dev \
+  llvm-libunwind-dev \
+  cmake \
+  curl-dev \
+  file \
+  libffi-dev \
+  libgit2-dev \
+  libressl-dev \
+  libssh2-dev \
+  llvm5-dev \
+  llvm5-test-utils \
+  python2 \
+  tar \
+  zlib-dev
+  
 
 ENV CC_x86_64_unknown_linux_musl=gcc