Move from dockerhub to quay.io

See https://github.com/jupyterhub/team-compass/issues/688
for context.

I've also added `QUAY_USERNAME` and `QUAY_PASSWORD` to environment
secrets, but *not* `env.REGISTRY`. I will do so once this gets
merged.

examples/service-fastapi/Dockerfile

@@ -1,9 +1,9 @@
-FROM jupyterhub/jupyterhub
+FROM quay.io/jupyterhub/jupyterhub
 
-# Create test user (PAM auth) and install single-user Jupyter 
+# Create test user (PAM auth) and install single-user Jupyter
 RUN useradd testuser --create-home --shell /bin/bash
 RUN echo 'testuser:passwd' | chpasswd
-RUN pip install jupyter 
+RUN pip install jupyter
 
 COPY app ./app
 COPY jupyterhub_config.py .