Add basic CI to build the subgraph in GA

[dapplion]

Adds a basic CI step to build and deploy the subgraph in a containerized environment.

Rationale

This setup can be extended by moving code from aragon/aragon-court to set up blockchain state in the ganache instance which could be queried later in a test.

Feel free to inspect the build logs in my fork https://github.com/dapplion/court-subgraph/runs/617879740?check_suite_focus=true#step:3:821

Recreate

The setup can be recreated locally with one line in a single terminal instance

docker-compose -f ./scripts/docker-compose.yml up --abort-on-container-exit --build --force-recreate --renew-anon-volumes --timeout 0
        # --abort-on-container-exit: Only the test service should exit.
        #   Then, the overall exit code will the the test service exit code
        # --build: Make sure to apply test container possible changes
        # --force-recreate: Make sure to clear ephemeral container
        # --renew-anon-volumes: Make sure postgress and ganache data deleted
        # --timeout 0: Speed up teardown

[facuspagnuolo]

This is great!