EQS-979: Add custom Datastore Emulator image

[VirajP1002]

What is the context of this PR?

The knarz/datastore is no longer available on DockerHub and the existing image is not compatible with the new Corporate MacBooks. Therefore, we require custom Datastore Emulator is required to provide the Google Cloud SDK Datastore Emulator.

The changes in this PR are an extension of this PR created by @pricem14pc (who created the Docker image & the corresponding README). The additional changes include:

• Updating our GitHub Actions workflows to build & push the Datastore Emulator image to our GAR
• Adding a new Make command to build the new image and adding this to the README
• Refactoring the folder name to align with the other folders.

How to review

• Ensure the image builds locally using make build-datastore-image and run the new image using the instructions in the README
• Push this to your own GAR in your own GCP project with a tag
• Update the docker-compose-dev.yml in Runner to point to your pushed Datastore image and ensure Runner works as before (where schemas can be opened and progressed).
  • Ensure you add the following environment var to the datastore service in the docker compose file: CLOUDSDK_CORE_PROJECT=local