Create docker.exs and docker-entrypoint + round out Dockerfile

At this point, the implementation is completely working and has been
tested running live and federating with other instances.

docker-entrypoint.sh

@@ -0,0 +1,14 @@
+#!/bin/ash
+
+set -e
+
+echo "-- Waiting for database..."
+while ! pg_isready -U ${DB_USER:-pleroma} -d postgres://${DB_HOST:-db}:5432/${DB_NAME:-pleroma} -t 1; do
+    sleep 1s
+done
+
+echo "-- Running migrations..."
+$HOME/bin/pleroma_ctl migrate
+
+echo "-- Starting!"
+exec $HOME/bin/pleroma start