Add x86_64-linux-alpine image

Addressing ghc#14502.

x86_64-linux-alpine/Dockerfile

@@ -2,29 +2,27 @@ FROM alpine:3.9
 
 RUN echo "Installing GHC build dependencies..." && \
     apk add --no-cache \
-        autoconf \
-        automake \
-        binutils-gold \
-        build-base \
-        coreutils \
-        cpio \
-        ghc \
-        linux-headers \
-        libffi-dev \
-        llvm5 \
-        musl-dev \
-        ncurses-dev \
-        perl \
-        python3 \
-        py3-sphinx \
-        zlib-dev \
-        wget \
-        cabal
+        autoconf=~2.69 \
+        automake=~1.16 \
+        binutils-gold=~2.31 \
+        build-base=~0.5 \
+        coreutils=~8.30 \
+        cpio=~2.12 \
+        ghc=~8.4 \
+        linux-headers=~4.18 \
+        libffi-dev=~3.2 \
+        musl-dev=~1 \
+        ncurses-dev=~6.1 \
+        python3=~3.6 \
+        py3-sphinx=~1.8 \
+        zlib-dev=~1.2 \
+        wget=~1.20 \
+        cabal=~2.2
 
 RUN cabal update
 RUN cabal install alex happy
 
-ENV ALEX=$HOME/.cabal/bin/alex
-    HAPPY=$HOME/.cabal/bin/alex
+ENV ALEX=/root/.cabal/bin/alex \
+    HAPPY=/root/.cabal/bin/happy \
     SPHINXBUILD=/usr/bin/sphinx-build-3