Change Dockerfile to Ubuntu with musl-tools, since we need glibc compatibility with host system

Author: strfry

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

@@ -1,11 +1,11 @@
-FROM alpine:3.8
+FROM ubuntu:16.04
 
-RUN apk update && \
-  apk add \
+RUN apt-get update && apt-get install -y \
+  musl-tools \
   curl \
   ca-certificates \
   perl \
   make \
   gcc
 
-ENV CC_x86_64_unknown_linux_musl=gcc
+ENV CC_x86_64_unknown_linux_musl=musl-gcc