Configuration to run in a docker container

[sirinsidiator]

This adds a Dockerfile to build the application and install ffmpeg so it can be run in a container. I have also updated the readme and added an example configuration for docker compose.

Inside the container the application is run as a non-priviledged user, so I have added some cli arguments to make it possible to work around any permission problems that may arise due to that:
--file-permissions (or -fp) and --directory-permissions (or -dp) to specify which mode is used to create files and directories.

In addition I also added another option --temp-directory (or -td) to specify a directory for temporary files which solves #175 for me.

Disclaimer: This is the first time I did anything with go, so please excuse if I made any obvious mistakes in the code. It compiles and runs fine though, so hopefully everything should be ok.

[Kethsar]

I'm not accepting commits to add Docker support directly into this repo. See #100 (comment)

If you make it so only changes regarding the new options are added, I'll be willing merge after testing it if it works.

[sirinsidiator]

Fair enough. I've created a new pull request for them. See #179 .

[sirinsidiator]

You could still add a short note to the readme regarding your stance though. I did look through the issues, but missed the pull request since github didn't include them in the search and I didn't actually expect there to be one.