WIP: Use GHC 8.6.5 as the minimum bootstrap compiler version

I have not yet figured out how to finagle the Alpine Linux builder to use GHC 8.6. For now, I'm opening this to see if my changes to the other builders were successful.

aarch64-linux-deb9/Dockerfile

@@ -25,8 +25,10 @@ RUN curl $LLVM_TARBALL | tar -xJC .. && \
     /opt/llvm6/bin/llc --version
 
 # GHC
-RUN curl http://downloads.haskell.org/~ghc/8.6.2/ghc-8.6.2-aarch64-ubuntu_bionic-linux.tar.xz | tar -xJ
-WORKDIR /ghc-8.6.2
+# Use 8.8.1 to bootstrap, since earlier releases are especially buggy on AArch64
+ENV GHC_VERSION 8.8.1
+RUN curl http://downloads.haskell.org/~ghc/$GHC_VERSION/ghc-$GHC_VERSION-aarch64-ubuntu_bionic-linux.tar.xz | tar -xJ
+WORKDIR /ghc-$GHC_VERSION
 RUN ./configure --prefix=/usr/local LLC=/opt/llvm6/bin/llc OPT=/opt/llvm6/bin/opt && \
     make install
 WORKDIR /
@@ -44,15 +46,16 @@ RUN curl $LLVM_TARBALL | tar -xJC .. && \
     $LLC --version
 
 # Cabal
+ENV CABAL_VERSION 3.0.0.0
 RUN git clone https://github.com/haskell/Cabal
 WORKDIR /Cabal
-RUN git checkout cabal-install-v2.4.1.0
+RUN git checkout cabal-install-v$CABAL_VERSION
 WORKDIR /Cabal/cabal-install
 RUN EXTRA_CONFIGURE_OPTS=--disable-optimization ./bootstrap.sh --global --no-doc
 
 RUN ls /Cabal/cabal-install/dist/build
 
-ENV PATH /home/ghc/.local/bin:/opt/cabal/2.2/bin:/opt/ghc/8.4.2/bin:$PATH
+ENV PATH /home/ghc/.local/bin:/opt/ghc/$GHC_VERSION/bin:$PATH
 
 # Create a normal user.
 RUN adduser ghc --gecos "GHC builds" --disabled-password