Set up Dependabot for signal-cli-rest-api

[tillprochaska]

Dependabot currently doesn't support Docker Compose, a possible fork around is to create a Dockerfile that contains only the following line

FROM user/repo:version

… and reference that Dockerfile in the compose config.

[mattwr18]

@roschaefer do you think you might get to this this quarter or should we move it to a future one?