Allow specification of additional conditional hosts (#9777)

Author: Eric Van Norman

Makefile.overrides.mk

@@ -18,4 +18,4 @@ CONTAINER_OPTIONS = -p 1313:1313 ${ADDITIONAL_CONTAINER_OPTIONS}
 # this repo is on the container plan by default
 BUILD_WITH_CONTAINER ?= 1
 
-CONDITIONAL_HOST_MOUNTS = --mount type=bind,source=/tmp,destination=/tmp ${}
+CONDITIONAL_HOST_MOUNTS = --mount type=bind,source=/tmp,destination=/tmp ${ADDITIONAL_CONDITIONAL_HOST_MOUNTS}