Enable pdb in a docker container (#6159)

Add config changes to docker-compose file to support pdb and update docs

docker-compose.yml

@@ -41,6 +41,8 @@ services:
       - mysql
       - redis
       - rabbitmq
+    stdin_open: true
+    tty: true
 
   frontend:
     container_name: frontend

docs/common_tasks.md

@@ -41,3 +41,11 @@ To add a new repository, the following steps are needed:
 
 For more information on adding a new GitHub repository, see
 [Adding a GitHub repository](submitting_data.md#adding-a-github-repository).
+
+## Debugging Tools
+
+You can use the Python Debugger ([pdb](https://docs.python.org/3.7/library/pdb.html)) in a Docker container.
+After starting a local Treeherder instance using [docker-compose](installation.md#server-and-full-stack-development),
+in a separate shell type `docker attach backend`. Then set a breakpoint in your file using either `import pdb; pdb.set_trace()`
+or `breakpoint()` (for Python v3.7+). The pdb debugger will start in that shell once the breakpoint has been triggered.
+For example, it can be triggered via refreshing the browser (localhost) if the view you're on calls an API with a breakpoint on it.