Disable docker container test from toplevel 'redo test'

It's not reliable on all versions of docker, and we haven't had time to
fix it yet, so just turn it off. It's not essential to redo.

Reported-by: fcsmith@gmail.com

docs/cookbook/container/all.do

@@ -49,6 +49,12 @@ if [ -n "$NO_SLOW_TESTS" ]; then
 	echo " -- NO_SLOW_TESTS is set."
 	echo "skipping debian image."
 	no_debian=1
+	# TODO(apenwarr): docker test isn't "slow" exactly, but error prone.
+	#  It might not work correctly on modern versions of docker.
+	#  Until we resolve this, just disable it from a toplevel 'redo test',
+	#  which sets NO_SLOW_TESTS.
+	echo "skipping docker test."
+	no_docker=1
 fi
 
 add() { targets="$targets $*"; }