Signal-cli

* improved mimetype detection when sending attachments (see 136)
* return timestamp of sent message in `/v2/send` endpoint (see 137)
* fixed debug mode output

* native mode now working on arm64 (thanks to webzit)
* properly handle port override in `AUTO_RECEIVE_SCHEDULE`

* updated signal-cli to v0.8.3
* added `AUTO_RECEIVE_SCHEDULE` parameter to `docker-compose.yml` (see https://github.com/bbernhard/signal-cli-rest-api#auto-receive-schedule for details)

* updated signal-cli to version 0.8.1
* reduced docker image size by ~20MB (thanks to haberda)
* reworked entrypoint script to make it easier for the Home Assistant addon to use the docker image
* use "--output=json" instead of "--json" when calling signal-cli receive (the latter one is deprectated)
* added missing information to swagger documentation

* updated signal-cli to version 0.8.1
* reduced docker image size by ~20MB (thanks to haberda)
* reworked entrypoint script to make it easier for the Home Assistant addon to use the docker image
* use "--output=json" instead of "--json" when calling signal-cli receive (the latter one is deprectated)

This is a pre-release. In case you want to give the docker image a try, use the following tag: `bbernhard/signal-cli-rest-api:0.60-dev`

* fixed ARM64 build