bst-here: Allow users to specify Docker image

Currently, the name of the Docker image is hardcoded in bst-here script.
This makes it harder for users to override it, which may be needed for
various reasons (custom plugins, private Docker registries etc.)

Add `-i` option to allow users to specify the base image. When `-i` is
not provided, default to `BST_HERE_IMAGE` environment variable if it's
set and otherwise to the current image -
`buildstream/buildstream-fedora`.

Also, re-order the command-line options in help text and source code in
alphabetical order to maintain sanity as the number of options is slowly
growing.

Part of #378.