Support for Docker and added non-default base directory argument

[iicky]

Hello! Love the Presence script and just implemented it in my own system using Docker Compose - hopefully this can help people save time when deploying on multiple systems. Here's what I added:

1. Added non-default base directory option as an argument to presence.sh (presence.sh -p path/to/dir)

2. As part of 1, moved PROCESS OPTIONS and HELP TEXT sections before LOAD PREFERENCES in presence.sh.

3. Added Docker support in the docker directory, providing two files for either x86 or Raspberry Pi architectures (I'm running on both).

4. Added installation instructions under the docker directory.

If you like it, feel free to implement it - just change the docker directory README.md to point from my repo to yours in the docker-compose.yaml file.

Thanks again!