Hi Martin,
I had a look at debian/rules, the problem seems to be in line 13:
BUILD_DOCKER = $(findstring $(shell . /etc/os-release; echo $${VERSION_ID:-unstable}),18.04 19.10)
In comparison to cockpit 215, you removed "10" from findstring's second argument, but "10" is also present in "19.10", so it is still matched by findstring and the resulting BUILD_DOCKER var is not empty as intended (on buster).
Replacing "findstring" with "filter" (which matches words) seems to do the trick though:
BUILD_DOCKER = $(filter $(shell . /etc/os-release; echo $${VERSION_ID:-unstable}),18.04 19.10)
I ran debuild with that change to debian/rules and it didn't build cockpit-docker package.
Cheers,
Bruno