feat: add config for tmp directory

[afbjorklund]

Closes #44

The default is "" (just like before), which means that it will use $TMPDIR (normally /tmp). Works fine with docker.

Added a feature to allow ~ (tilde) in path, so that you can copy/paste config without having to expand $HOME...

Example:

    "docker": {
         "bin": "nerdctl.lima",
         "tmp": "~/.local/tmp"
    },

It will also create the directory if it does not exist, since these non-standard directories (in ~/.local) might not.