All persistent data (configuration, application data) is stored in the data directory specified by the `-d` command line argument, defaulting to `~/.config/signald` if unspecified.
The signald socket is created at the location specified by the `-s` command line argument, defaulting to `/var/run/signald/signald.sock` if unspecified.

This updates the Docker image to:
  -  use `/signald` (as is done in the image used by mautrix-signal) for application data, configuration and the socket location
  - declares a mount point
  - use Java 11 instead of Java 8 by default
  - start in verbose mode