Add extra_filerefs to sls_build

[LukeCarrier]

What does this PR do?

Adds an extra_filerefs option to docker.sls_build, consistent with the salt-ssh --extra-filerefs option. This enables developers to include requirements of templates (e.g. a macros.sls file) in the thin bundle sent to the container.

What issues does this PR fix or reference?

• salt-ssh jinja TemplateNotFound  #9878 -- original issue against salt-ssh
  • Adds support for extra_filerefs=salt://path1,salt://path2 CLI arg to inc... #13794 -- add CLI argument
  • Allow extra file refs to be defined in opts and saltfile per #9878 #16733 -- add extra_filerefs option to salt.client.ssh.state.lowstate_file_refs

Previous Behavior

Any missing template dependencies would fail with a TemplateNotFound error like the following:

Traceback (most recent call last):
  File "/tmp/salt.docker.[snip]/pyall/salt/utils/templates.py", line 388, in render_jinja_tmpl
    output = template.render(**decoded_context)
  File "/tmp/salt.docker.[snip]/pyall/jinja2/environment.py", line 1008, in render
    return self.environment.handle_exception(exc_info, True)
  File "/tmp/salt.docker.[snip]/pyall/jinja2/environment.py", line 780, in handle_exception
    reraise(exc_type, exc_value, tb)
  File "<template>", line 1, in top-level template code
  File "/tmp/salt.docker.[snip]/pyall/salt/utils/jinja.py", line 140, in get_source
    raise TemplateNotFound(template)
TemplateNotFound: web-base/macros.sls

New Behavior

Including extra_filerefs, e.g. as follows, allows the states to complete as the necessary dependencies are included in the thin archive transferred to the container:

$ salt-call \
        --local --file-root ~/salt --pillar-root ~/pillar \
        docker.sls_build container base=base mods=some-state,some-other-state \
        extra_filerefs=salt://some-state/some-dep.jinja

Tests written?

No -- I'm not sure how best to test this. I'm happy to try if I can get an idea of where to start?

Commits signed with GPG?

No