docker: fix root squashed data dirs

if we use some root squashed volume mappings in docker
uid=0 is actually uid=nobody
thus FS operations with uid=0 will fail miserably
and overall it's a strange idea to create dirs under root when
DO_NOT_CHOWN is in effect...

docker/server/entrypoint.sh

@@ -65,7 +65,12 @@ do
     # check if variable not empty
     [ -z "$dir" ] && continue
     # ensure directories exist
-    if ! mkdir -p "$dir"; then
+    if [ "$DO_CHOWN" = "1" ]; then
+      mkdir="mkdir"
+    else
+      mkdir="$gosu mkdir"
+    fi
+    if ! $mkdir -p "$dir"; then
         echo "Couldn't create necessary directory: $dir"
         exit 1
     fi