Improve documentation on how to run tests

Also update gitignore / dockerignore

.dockerignore

@@ -2,3 +2,4 @@
 Dockerfile
 docker-compose*.yml
 .vagrant/
+codecov/

.gitignore

@@ -13,3 +13,4 @@ cache/
 lib/
 log/
 .vagrant/
+codecov/

docs/Docker image.rst

@@ -0,0 +1,34 @@
+Docker image
+************
+
+We provide a docker image (defined in :gh_blob:`Dockerfile`) that can be used to
+easily get an instance of CMS running locally with all the necessary
+dependencies. We also provide a :gh_blob:`docker-compose.test.yml` files that
+uses said docker image to run the tests.
+
+.. _docker-image_running-tests:
+
+Running tests
+=============
+
+First you need to build the image:
+
+.. sourcecode:: bash
+
+    sudo docker-compose -f docker-compose.test.yml build cms_test
+
+Then you can run the tests:
+
+.. sourcecode:: bash
+
+    sudo docker-compose -f docker-compose.test.yml run --rm cms_test
+
+This command will create a ``cms_test_db`` container for the database which
+**will not** be automatically deleted, and a ``cms_test`` container for CMS
+which will be automatically deleted (because of the ``--rm`` flag) upon exiting.
+
+To delete the ``cms_test_db`` container after testing you can run:
+
+.. sourcecode:: bash
+
+    sudo docker rm -f cms_test_db

docs/index.rst

@@ -5,6 +5,7 @@ Welcome to CMS's documentation!
    :maxdepth: 2
 
    Introduction
+   Docker image
    Installation
    Running CMS
    Data model