Add comment about Dockerfiles vs config files

docs/source/config_files.txt

@@ -15,6 +15,11 @@ Below is a list of supported configuration files (roughly in the order of build
 This will be treated as a regular Dockerfile and a regular Docker build will be performed.
 The presence of a Dockerfile takes priority over and ignores all other build behavior
 specified in other configuration files.
+
+In the majority of cases, providing your own Dockerfile is not necessary as the base
+images provide core functionality, compact image sizes, and efficient builds. We recommend
+trying the other configuration files before deciding to use your own Dockerfile.
+
 See the `Binder Documentation <https://mybinder.readthedocs.io/en/latest/dockerfile.html>`_ for
 best-practices with Dockerfiles.