Document loading db schema in docker dev/test environments

[mcritchlow]

Local Checklist

• Code style checked?
• QA-ed locally?
• Rebased with master branch?
• Documentation updated (if needed)?

What does this PR do?

• Documents the change to use db:schema:load instead of db:migrate when using Docker for dev/test work. This is not only faster, but also ensures the db/schema.rb file doesn't get updated, which should really only happen when we add new migrations.
• Documents removing docker volumes along with containers.
• Updates CircleCI config to use db:schema:load for consistency (and minor speed gain)

Why are we doing this? Any context of related work?

• Prevent accidental commits of modified db/schema.rb file. And speed.

Where should a reviewer start?

• docker/README.md

Manual testing steps?

• Feel free to run this locally and confirm if you're up for it.

@ucsdlib/developers - please review

[hweng]

👍

[VivianChu]

👍

[mcritchlow]

@ucsdlib/developers - thanks for your patience. got circleci using the new db:schema:load command as well now.

[coveralls]

Coverage remained the same at 66.655% when pulling 14773aa on docker-db-schema into a6bf0cb on develop.

[lsitu]

👍