ci: Add GH action for building/pushing API Docker image

[yondonfu]

Changes in this PR:

• Add a GH action that automatically builds and pushes the livepeer/verifier and livepeer/verifier-api images to the Livepeer DockerHub every time a new commit is pushed to the repo allowing the images to be directly pulled from the registry without having to build them locally.

  • The latest tag is used for the livepeer/verifier image
  • The branch name (with some symbol substitution required for Docker tag names) is used for the livepeer/verifier-api image

• Updates all references to image names in the repo to follow the <REPO_NAME>/<IMAGE_NAME> convention.

Fixes #92

[yondonfu]

Added 9568c44 to fix a youtube-dl dependency issue.