Manually install unreleased version of docker-py

Brings in a fix for docker/docker-py#3240,
until docker-py is released upstream.

Ref jupyterhub#117

Dockerfile

@@ -4,6 +4,11 @@ RUN apk add --no-cache curl build-base python3 python3-dev py3-pip
 
 RUN python3 -m pip install --upgrade wheel setuptools
 
+# Manually downgrade version of docker-py
+# Until a fix for https://github.com/docker/docker-py/issues/3240
+# is released, we want to use an older version of docker-py
+RUN pip install 'docker<7.0'
+
 # https://stackoverflow.com/a/41651363/1695486
 RUN apk add --no-cache curl curl-dev
 COPY create_docker_image.sh /create_docker_image.sh