Use alpine 3.10 in building linux binary.

For some reason the bulid fails with alpine 3.11,
with a linker error finding -lz.  See #6003.
It would be good to figure out how to make it work
with alpine 3.11.

linux/Dockerfile

@@ -1,5 +1,5 @@
 # USE ALPINE LINUX
-FROM alpine
+FROM alpine:3.10
 RUN apk update
 RUN apk add alpine-sdk git ca-certificates ghc gmp-dev zlib-dev bash dpkg fakeroot cabal
 RUN mkdir -p /usr/src/