Package Pinboard as Docker container

[gggeek]

Given its "web app" nature, it seems to be a good target to be made available as container.

Especially considering that the Symfony version it is based on is now a bit crusty, and it might be hard to get it installed along other php code.

The container could be built via GitHub Actions, and stored in the GitHub registry.

A couple of env vars would be needed to allow it to talk to a Pinba server at a specific host/port. Upgrading to a more recent Symfony version would make it easier to use config based on env vars