[GH-55] Docker multi-stage build

[OneCricketeer]

Summary

After Gradle clean

$ docker build --no-cache -t streaming-connect .
...
=> => transferring context: 25.73kB

Related Issue

Closes #55

Changes

Update Dockerfile and .dockerignore to not use copied-in artifacts and rather build within the container

Benefits

1. Consistent build versions. I have JDK 17 as default, but the runtime image wants JRE 11
2. Faster build contexts, with the tradeoff of pulling more images (which would be cached later)

How Has This Been Tested?

Local builds

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• [x] I have signed the Adobe Open Source CLA.
• [x] My code follows the code style of this project.
• [x] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [ ] I have read the CONTRIBUTING document.
• [ ] I have added tests to cover my changes.
• [x] All new and existing tests passed.