Quick: Add empty Core cms directory

[wesleyboar]

Dependencies

• Quick: Create & Skip empty Core cms directory Core-CMS#176

Overview

Support core-cms as CUSTOM_ASSET_DIR in image build configuration (even if Core has no assets).

This will allow us to combine dockerhub_repo and CUSTOM_ASSET_DIR into one parameter.

Related Tickets

• FP-981

Testing

0. Build new docker image using core-cms for both.
   • You may use taccwma/core-cms:7d3351b.
1. Follow Test New CMS Site instructions.
   • If testing on a local standalone CMS, be sure you're on the matching branch for Core-CMS.
2. (In Step 2 of Test New CMS Site) Set CMS to run as core-cms.
3. Load a page that uses the Core fullwidth.html template.
4. Ensure background is white.*†

* If background is not white, and you are testing with local image on local standalone CMS, be sure you run npm run build.
† Because example-cms, previous "Core" build has yellow background.

Resources

• remote docker image: taccwma/core-cms:7d3351b

[wesleyboar]

I am merging without explicit peer approval, because the parent PR is ready for merge.