chore(nimbus): Remove file caching from Dockerfile

[brennie]

Because:

• file caching was added in fix #5946 chore(project): cache docker layers in dockerhub #5948 (see Cache docker layers between ci and local dev #5946) to enable remote caching;
• we are no longer using remote caching;
• this step adds complexity to our build process; and
• this step may be triggering unnecessary container rebuilds

this commit:

• removes file caching.

Fixes #11207.

[jaredlockhart]

Bye bye file loader 👋 thnx @brennie