Add XLoader image to run in it's own container, separate from the CKAN UI container

[kowh-ai]

Fixes #66

This PR attempts to create an XLoader image to be used as a replacement for DataPusher.

However it currently has a problem in that the callback to the ckan container uses the resource data URL. This means if the URL is something like http://localhost:5000/dataset/testset20/resource/26004e4f-d13c-49f3-b370-bc38806050fa XLoader will try to connect to the ckan container running on localhost:5000 which is the xloader container rather than the ckan container

There needs to be a modification to the XLoader extension itself to allow similar environment variable to DataPusher ie: ckan.datapusher.callback_url_base. This would allow control of over what URL xloader would use to communicate back to the ckan container

It seems these are currently the only xloader configuration parameters available:
ckanext.xloader.jobs_db.uri
ckanext.xloader.api_token
ckanext.xloader.formats
ckanext.xloader.max_content_length
ckanext.xloader.use_type_guessing
ckanext.xloader.strict_type_guessing
ckanext.xloader.max_type_guessing_length
ckanext.xloader.parse_dates_dayfirst
ckanext.xloader.parse_dates_yearfirst
ckanext.xloader.job_timeout
ckanext.xloader.ignore_hash
ckanext.xloader.max_excerpt_lines
ckanext.xloader.ssl_verify
ckanext.xloader.clean_datastore_tables

[amercader]

This shouldn't be difficult to implement in xloader, I've put down the work needed here:

ckan/ckanext-xloader#226