Remove legacy docker-compose.yml

Plugin already uses wp-env for local development. This removes the
old Docker setup that was superseded by wp-env.

- Remove docker-compose.yml
- Update .distignore

Author: pfefferle

.distignore

@@ -9,12 +9,11 @@
 package.json
 composer.json
 composer.lock
-Gruntfile.js
 push.sh
 phpunit.xml
 phpunit.xml.dist
 phpcs.xml
 .travis.yml
 .distignore
 .gitignore
-docker-compose.yml
+.wp-env.json