Dockerfile: build with the HEAD version of cabal-install

This is necessary to avoid haskell/cabal#6125.
Fixes haskell/cabal#6125 at last.

Author: peti

Dockerfile

@@ -10,35 +10,43 @@
 # Docker> # hackage-server run --static-dir=datafiles
 #
 
+# Set up the system environment.
 FROM ubuntu
-
 RUN apt-get update
 RUN apt-get install -y software-properties-common
 RUN apt-add-repository ppa:hvr/ghc
 RUN apt-get update
-RUN DEBIAN_FRONTEND=noninteractive apt-get install -y unzip libicu-dev postfix
 RUN DEBIAN_FRONTEND=noninteractive apt-get install -y ghc-8.2.2 cabal-install-2.4
 ENV PATH /opt/ghc/bin:$PATH
 RUN cabal v2-update
-RUN apt-get install -y zlib1g-dev libssl-dev
+
+# Install current HEAD version of cabal-install.
+RUN DEBIAN_FRONTEND=noninteractive apt-get install -y curl zlib1g-dev
+WORKDIR /tmp
+RUN curl --silent -L https://github.com/haskell/cabal/tarball/dc138034d469973fee43af86764567e4202ca84b | tar xz
+WORKDIR /tmp/haskell-cabal-dc13803
+RUN cabal v2-install -j cabal-install
+ENV PATH /root/.cabal/bin:$PATH
+
+# Build hackage-server.
+RUN DEBIAN_FRONTEND=noninteractive apt-get install -y zlib1g-dev libssl-dev unzip libicu-dev postfix
 RUN mkdir /build
 WORKDIR /build
 ADD hackage-server.cabal cabal.project ./
-RUN cabal v2-build --only-dependencies --enable-tests -j
-RUN cabal v2-install hackage-repo-tool
-ENV PATH /root/.cabal/bin:$PATH
+RUN cabal v2-build --only-dependencies --enable-tests
+RUN cabal v2-install --help
+RUN cabal v2-install --installdir=/root/.cabal/bin hackage-repo-tool
 ADD . ./
 RUN hackage-repo-tool create-keys --keys keys
 RUN cp keys/timestamp/*.private datafiles/TUF/timestamp.private
 RUN cp keys/snapshot/*.private datafiles/TUF/snapshot.private
 RUN hackage-repo-tool create-root --keys keys -o datafiles/TUF/root.json
 RUN hackage-repo-tool create-mirrors --keys keys -o datafiles/TUF/mirrors.json
-RUN cabal v2-build
 # tests currently don't pass: the hackage-security work introduced some
 # backup/restore errors (though they look harmless)
 # see https://github.com/haskell/hackage-server/issues/425
 #RUN cabal v2-test
-RUN cabal v2-install -j .
+RUN cabal v2-install --installdir=/root/.cabal/bin all
 
 # setup server runtime environment
 RUN mkdir /runtime