Disable optimization when building Cabal

The Cabal build is enormous and constitutes the majority of the build time of the AArch64 and armv7 images. This cuts the Cabal build time down by 60%.

aarch64-linux-deb9/Dockerfile

@@ -48,7 +48,7 @@ RUN git clone https://github.com/haskell/Cabal
 WORKDIR /Cabal
 RUN git checkout cabal-install-v2.4.1.0
 WORKDIR /Cabal/cabal-install
-RUN ./bootstrap.sh --global --no-doc
+RUN EXTRA_CONFIGURE_OPTS=--disable-optimization ./bootstrap.sh --global --no-doc
 
 RUN ls /Cabal/cabal-install/dist/build