Disable upstream PGDG apt source before update

Upstream catmaid/catmaid-standalone bakes the apt.postgresql.org
PGDG repo. PGDG retired focal-pgdg when Ubuntu 20.04 hit standard
EOL, so the Release file is gone and apt-get update aborts with
'does not have a Release file', failing the whole build.

Postgres is already installed in the base image, so we don't need
PGDG to be reachable at build time. Delete the PGDG source file
before running update. r-base resolves fine from focal main once
PGDG is out of the way.

Author: Robbie1977

Dockerfile

@@ -32,7 +32,14 @@ RUN chmod -R 777 /opt/VFB
 
 RUN chmod +x /opt/VFB/*.sh
 
-RUN apt-get update && apt-get install -y r-base
+# Upstream catmaid/catmaid-standalone bakes the PostgreSQL Global
+# Development Group (PGDG) apt repo, but PGDG retired focal-pgdg when
+# Ubuntu 20.04 hit standard EOL — its Release file is gone, so apt-get
+# update aborts. Postgres is already installed in the base image, so we
+# just disable the PGDG source before running update.
+RUN rm -f /etc/apt/sources.list.d/pgdg.list /etc/apt/sources.list.d/postgresql.list \
+    && apt-get update \
+    && apt-get install -y r-base
 
 ENV INSTANCE_MEMORY=65000