always autoamtically create the 'assets/' subdirectory
issue warnings for common mistakes in snippets

- if a snippet definition has an unknown section
- if a snippet's dockerfile entry starts with an
  invalid Dockerfile command
add the 'logrotate' snippet
update README.md

- use COPY/ADD insteaf of ADD
- note on YAML literal blocks
- note on secrets
implement the 'set_root_password' snippet

- this is an example of using secret files
cleanup mysql_* snippets using secrets and mysql_{start,stop}
clean up warning/logging logic
merge Drupal/Apache related snippets into one

- introduces: drupal_apache_2.2_site
- removes: apache_2.2._site,
           drupal_settings.local.php,
           drupal_docroot_permissions
mark PHONY targets as such in the generated Makefile, closes #4
Include `make/*.mk' in generated Makefile for custom targets, closes #5