feat: filter containers seen by docker-gen

[buchdag]

The PR add the -container-filter option to filter the containers that docker-gen will see.

The option work pretty much like -notify-filter and can be used multiple times to combine filters with AND.

This:

docker-gen \
    -container-filter status=running \
    -container-filter label=com.github.nginx-proxy.nginx \
    ./some/template.tmpl ./result.ext

will result in docker-gen only seeing running container that possess the label com.github.nginx-proxy.nginx.

Closes #276

Unfortunately Docker filter options for containers does not provide negative filtering, so #117 and #252 won't be entirely fixed by this.

Thanks @tarasov65536 for the inspiration.