Migrated data dirs to mount instead of docker volume

Closes https://linear.app/ghost/issue/ENG-2413

- This makes migration from standard `ghost-cli` installs to Docker a bit easier and also tries to prevent people from accidentally pruning their Docker volumes are removing data

.env.example

@@ -2,6 +2,12 @@
 
 DOMAIN=example.com
 
+# Location to store uploaded data
+UPLOAD_LOCATION=./data/ghost
+
+# Location for database data
+MYSQL_DATA_LOCATION=./data/mysql
+
 DATABASE_ROOT_PASSWORD=reallysecurerootpassword
 DATABASE_USER=optionalusername
 DATABASE_PASSWORD=ghostpassword