Add apache to docker and fixup docs

Re #52 (comment 10712)

This does a few things:

• switch from single docker container to docker-compose with two containers
  • a ruby one that watches and builds the site into _site
  • an httpd one that mounts _site into /usr/local/apache2/htdocs/ and serves them
• httpd config is in docker/web/httpd.conf which gets mounted into /usr/local/apache2/conf/httpd.conf with a few changes from default:
  • extra modules enabled: mod_deflate + mod_negotiation
  • add config --> Options Indexes Includes FollowSymLinks SymLinksifOwnerMatch ExecCGI MultiViews
  • add config --> AllowOverride All
• .htaccess is base dir
  • is copied to _site by jekyll build
  • used by httpd container
• puts ruby deps into a docker volume
  • ... they don't appear on your system
  • docker-compose down -v will remove everything docker related
• fixes up README.md to reflect reality again
• removes temporary fix for sass (as the normal one works again now)