add local container testing docs

Author: gilesknap

docs/developer/how-to/test-container.rst

@@ -0,0 +1,25 @@
+Container Local Build and Test
+==============================
+
+CI builds a runtime container for the project. The local tests
+checks available via ``tox -p`` do not verify this because not
+all developers will have docker installed locally.
+
+If CI is failing to build the container, then it is best to fix and
+test the problem locally. This would require that you have docker
+or podman installed on your local workstation.
+
+In the following examples the command ``docker`` is interchangeable with
+``podman`` depending on which container cli you have installed.
+
+To build the container and call it ``test``::
+
+    cd <root of the project>
+    docker build -t test .
+
+To verify that the container runs::
+
+    docker run -it test --help
+
+You can pass any other command line parameters to your application
+instead of --help.

docs/developer/index.rst

@@ -32,6 +32,7 @@ side-bar.
             how-to/lint
             how-to/update-tools
             how-to/make-release
+            how-to/test-container
 
         +++