Use cache during Docker image build process in Github Actions

[abitmore]

User Story

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

https://dev.to/dtinth/caching-docker-builds-in-github-actions-which-approach-is-the-fastest-a-research-18ei

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)
• 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

• Evaluate / Prioritize Feature Request
• Refine User Stories / Requirements
• Define Test Cases
• Design / Develop Solution
• Perform QA/Testing
• Update Documentation