Adopt `binderhub`

[trungleduc]

User facing changes

This PR allows tljh-repo2docker to use binderhub service as the image build backend. By default, it still uses the local docker engine to build image via repo2docker, but if the binderhub_url is specified, it will switch to using binderhub service instead.

New configurations

• binderhub_url: The optional URL of the binderhub service. If available, tljh-repo2docker will use this service to build images.
• db_url: The database's connection string. tljh-repo2docker needs a database to store the image metadata. By default, it will create a sqlite database in the starting directory of the service. To use other databases (PostgreSQL or MySQL), users need to specify the connection string via this config and install the additional drivers (asyncpg or aiomysql).

New environment dialog when using binderhub service

[!NOTE] The binderhub build backend does not support adding custom build arguments and private repository credentials from the interface.

Code changes

• New database migration and management tools.
• New handler to build images via binderhub service.
• New handler to follow the build process.
• Updated interfaces for building images.
• New unit tests and UI tests for binderhub build backend.

[jtpio]

Nice, thanks @trungleduc for working on this!

Also cc @yuvipanda if you would like to help review and / or provide feedback. This should help with https://github.com/2i2c-org/binderhub-service/issues/78.

[yuvipanda]

This is super exciting to me. How can I help? Would it be via code review? Or something else?

[trungleduc]

Hi @yuvipanda, a quick look at the implementation would be super helpful for us.

[jtpio]

Also @yuvipanda if you would like to start testing this on some deployments and have some feedback or issues, please don't hesitate to reach out!