Add make validate-via-docker

Green on my machine

.dockerignore

+# Note: some cabal-testsuite tests need .git, so we cannot ignore that.
+
+cabal.project.local
+.ghc.environment.*
+
+**/.hpc
+**/*.hi
+**/*.o
+**/*.p_hi
+**/*.prof
+**/*.tix
+
+**/*.sw*
+
+**/Setup
+**/*.dist
+
+Cabal/dist/
+Cabal/.python-sphinx-virtualenv/
+Cabal/tests/Setup
+Cabal/Setup
+Cabal/source-file-list
+
+cabal-install/dist/
+cabal-install/Setup
+cabal-install/source-file-list
+
+.github/
+dist-newstyle/
+dist-newstyle-*/
+
+Cabal/.python-sphinx-virtualenv

Makefile

@@ -105,3 +105,6 @@ cabal-install-test:
 	$(CABALBUILD) -j3 cabal-tests cabal
 	rm -rf .ghc.environment.*
 	cd cabal-testsuite && `cabal-plan list-bin cabal-tests` --with-cabal=`cabal-plan list-bin cabal` --hide-successes -j3 ${TEST}
+
+validate-via-docker:
+	docker build -t cabal-validate -f validate.dockerfile .

validate.dockerfile

+FROM haskell:8.6.5
+
+# We need prof GHC for some tests
+RUN     apt-get update
+RUN     apt-get install ghc-8.6.5-prof
+
+# Install cabal-plan
+RUN     cabal v2-update
+RUN     cabal v2-install cabal-plan --constraint 'cabal-plan ^>=0.6'
+
+# Validate
+WORKDIR /app
+COPY    . /app
+RUN     sh ./validate.sh -w ghc-8.6.5