Add libc-dev

[jhayotte]

After the last commit https://github.com/znly/docker-protobuf/commit/2fce12f5dd57f8efcab02aec118663b69f0ffdaf we get compiling error. It's due to the upgrade of the alpine image.

/usr/lib/go/pkg/tool/linux_amd64/link: running gcc failed: exit status 1
/usr/lib/gcc/x86_64-alpine-linux-musl/6.3.0/../../../../x86_64-alpine-linux-musl/bin/ld: cannot find Scrt1.o: No such file or directory
/usr/lib/gcc/x86_64-alpine-linux-musl/6.3.0/../../../../x86_64-alpine-linux-musl/bin/ld: cannot find crti.o: No such file or directory
/usr/lib/gcc/x86_64-alpine-linux-musl/6.3.0/../../../../x86_64-alpine-linux-musl/bin/ld: cannot find -lpthread
/usr/lib/gcc/x86_64-alpine-linux-musl/6.3.0/../../../../x86_64-alpine-linux-musl/bin/ld: cannot find -lssp_nonshared
collect2: error: ld returned 1 exit status

Adding package libc-dev resolves build problems.

It's a known issue that already occurred at ETSY https://github.com/etsy/hound/pull/237/commits/7fad470f9c4b9cbd642d9635bf79571abfa17b5d

For now, I added this package in my custom image however, it would be nice if you would add it directly here :)