Update Docker build to use Debian as a base

This makes it easier to do Haskell builds and incorporate external
tools, even though it results in a slightly bigger image.

Dockerfile

@@ -1,9 +1,11 @@
-FROM alpine:edge AS build
-RUN adduser -D cryptol \
-    && apk add -X http://dl-cdn.alpinelinux.org/alpine/edge/testing z3 \
-    && rm /usr/lib/libz3.so* \
-    && apk add build-base cabal ghc wget ncurses-dev \
-    && su -c 'cabal update' cryptol
+FROM haskell:8.6 AS build
+RUN apt-get update \
+    && apt-get install -y wget libncurses-dev unzip \
+    && wget https://github.com/Z3Prover/z3/releases/download/z3-4.8.4/z3-4.8.4.d6df51951f4c-x64-debian-8.11.zip \
+    && unzip z3*.zip \
+    && mv z3-*/bin/z3 /usr/local/bin
+RUN useradd -m cryptol \
+    && su -c '/opt/cabal/bin/cabal v2-update' cryptol
 COPY --chown=cryptol:cryptol . /cryptol
 USER cryptol
 WORKDIR /cryptol
@@ -14,22 +16,23 @@ ARG IGNORE_EXPECTED="--ignore-expected"
 ARG CABAL_BUILD_FLAGS="-j"
 ARG CABAL_INSTALL_FLAGS="${CABAL_BUILD_FLAGS}"
 RUN make tarball
+ARG TIME=""
 RUN make test
 RUN mkdir -p rootfs/"${CRYPTOLPATH}" \
     && cp -r lib/* rootfs/"${CRYPTOLPATH}" \
     && mkdir -p rootfs/usr/local \
     && rm -r cryptol-*-Linux-*_unknown/share/doc \
-    && mv cryptol-*-Linux-*_unknown/* rootfs/usr/local
+    && mv cryptol-*-Linux-*_unknown/* rootfs/usr/local \
+    && mv /usr/local/bin/z3 rootfs/usr/local/bin/z3
 USER root
 RUN chown -R root:root /cryptol/rootfs
 
-FROM alpine:edge
+FROM debian:stretch-slim AS build
+RUN apt-get update \
+    && apt-get install -y libgmp10 libffi6 wget libncurses5 unzip
 COPY --from=build /cryptol/rootfs /
-RUN adduser -D cryptol \
+RUN useradd -m cryptol \
     && chown -R cryptol:cryptol /home/cryptol \
-    && apk add -X http://dl-cdn.alpinelinux.org/alpine/edge/testing z3 \
-    && rm /usr/lib/libz3.so* \
-    && apk add gmp libffi musl ncurses-libs \
     && rm -r /var/cache/apk/*
 USER cryptol
 ENTRYPOINT ["/usr/local/bin/cryptol"]