[CI] Docker layer caching

[4TT1L4]

Problem:

• All the layers of the docker image are rebuilt on every build when the CI build is run on GHA.

Solution:

• Setup docker layer caching for the GitHub Actions CI build.

[4TT1L4]

Implemented:

https://github.com/geniusyield/smart-order-router/blob/b585bc04d42bd60e2e9c853835ac8029f657279d/.github/workflows/build.yml#L29-L30

And tested:

• The build time for a new branch created from main is down to 6 minutes 12 seconds from the earlier build time around 1 hour.

See the following build for example: https://github.com/geniusyield/smart-order-router/actions/runs/6509150911