Hi! I've come across this same issue and I fixed it using ACL. The point is knowing what subuid's would sbuild use and experimentally I found that that it may use 166535 and 165536. The first is used for root user in the namespace and the latter for operations as sbuild user in the sbuild namespace.
The command I used is: setfacl -m u:166535:rwX -m u:165536:rwx $HOME/.cache/ccache I'm about to include this information in the sbuild page in the wiki. Note: $HOME/.cache/ccache is "traversable" by sbuild users in namespace. For this you can either use the ACL approach or the traditional chmod. Best Regards.
