Use cache during Docker image build process in Github Actions

User Story

As a developer I want the Docker build workflow in Github Actions to run faster so that my development work becomes easier.

Note that production images should be built with no-cache to always include the latest security updates. https://pythonspeed.com/articles/docker-cache-insecure-images/

Impacts Describe which portion(s) of BitShares Core may be impacted by your request. Please tick at least one box.

[ ] API (the application programming interface)
[x] Build (the build process or something prior to compiled code)
[ ] CLI (the command line wallet)
[ ] Deployment (the deployment process after building such as Docker, Travis, etc.)
[ ] DEX (the Decentralized EXchange, market engine, etc.)
[ ] P2P (the peer-to-peer network for transaction/block propagation)
[ ] Performance (system or user efficiency, etc.)
[ ] Protocol (the blockchain logic, consensus, validation, etc.)
[ ] Security (the security of system or user data, etc.)
[ ] UX (the User Experience)
[ ] Other (please add below)

Additional Context (optional) Add any other context about your request here.

CORE TEAM TASK LIST