Use Python 3.8 in Docker images by default (#8698)

This bumps us closer to current Python without going all the way to 3.9.

Fixes #8674

Signed-off-by: Dan Callahan <danc@element.io>

changelog.d/8698.misc

@@ -0,0 +1 @@
+Use Python 3.8 in Docker images by default.

docker/Dockerfile

@@ -11,7 +11,7 @@
 #    docker build -f docker/Dockerfile --build-arg PYTHON_VERSION=3.6 .
 #
 
-ARG PYTHON_VERSION=3.7
+ARG PYTHON_VERSION=3.8
 
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 ### Stage 0: builder