Add docker:emptydb yarn command, to blow away the dev dbhub db

Useful for creating an empty dbhub database, ready to have an
existing populated snapshot loaded into it for dev purposes

package.json

@@ -32,6 +32,7 @@
     "docker:attach": "docker attach dbhub-build",
     "docker:build": "docker build --tag dbhub-build docker",
     "docker:debug": "docker exec -it dbhub-build /bin/sh /usr/local/bin/debug.sh",
+    "docker:emptydb": "docker exec -it dbhub-build pkill dbhub- && docker exec -itu postgres dbhub-build sh -c 'dropdb dbhub && createdb -O dbhub dbhub'",
     "docker:exec": "docker exec -it dbhub-build /bin/sh",
     "docker:github": "docker run -itd --rm --name dbhub-build -p 9443-9445:9443-9445/tcp --mount type=bind,src=\"$(pwd)\",target=/dbhub.io dbhub-build:latest",
     "docker:psql": "docker exec -it dbhub-build psql -U dbhub dbhub",