Enable continuous integration

Using the official Coq Docker images kindly provided by the Coq community:

	https://github.com/coq-community/docker-coq/wiki/CI-setup

Using the following CI template as a starting point:

	https://gitlab.com/erikmd/docker-coq-gitlab-ci-demo-1/blob/master/.gitlab-ci.yml

.gitlab-ci.yml

+# Taken from https://gitlab.com/erikmd/docker-coq-gitlab-ci-demo-1/blob/master/.gitlab-ci.yml
+
+stages:
+  - build
+
+.build:
+  stage: build
+  image: coqorg/${CI_JOB_NAME}
+  before_script:
+    - if [ -n "${COMPILER_EDGE}" ]; then opam switch ${COMPILER_EDGE} && eval $(opam env); fi
+    - opam update -y
+    # NOTE: we currently fix the mathcomp version to 1.8.0 here because
+    #       version 1.9.0 causes a build failure that still needs to be
+    #       investigated.
+    - opam install -y -j ${NJOBS} coq-mathcomp-ssreflect.1.8.0
+    - opam config list
+    - opam repo list
+    - opam list
+  script:
+    - sudo chown -R coq:coq "$CI_PROJECT_DIR"
+    - ./create_makefile.sh
+    - make -j ${NJOBS}
+    - make validate
+
+coq:8.9:
+  extends: .build
+
+# Currently does not work with fixed ssreflect version
+#coq:dev:
+#  extends: .build