new: provide ``docker_get_uid`` for charms.

bin/compose-core

@@ -1685,6 +1685,18 @@ init-config-add() {
 export -f init-config-add
 
 
+docker_get_uid() {
+    local service="$1" user="$2" uid
+    uid=$(cached_cmd_on_base_image "$service" "id -u \"$user\"") || {
+        debug "Failed to query for '$user' uid in ${DARKYELLOW}$service${NORMAL} base image."
+        return 1
+    }
+    info "uid from ${DARKYELLOW}$service${NORMAL} for user '$user' is '$uid'"
+    echo "$uid"
+}
+export -f docker_get_uid
+
+
 logstdout() {
     local name="$1"
     sed -r 's%^%'"${name}"'> %g'