Add Github action that creates the ephemeral cluster docker image

[rach-id]

As of the discussion under #415 (comment) , after creating a new Dockerfile and made it the official repository docker image, and renamed the existing one to Dockerfile_ephemeral, as it is more shaped for the https://github.com/celestiaorg/ephemeral-cluster . We will need to add a new Github action that creates another docker image called: ghcr.io/celestiaorg/celestia-app-ephemeral:latest which will build the Dockerfile_ephemeral image.