docker: remove default ACC value, protect var expansion

The default makes little sense after 9b19b08e06,
but the feature might still be useful.

.docker/privnet-entrypoint.sh

@@ -2,15 +2,11 @@
 
 BIN=/usr/bin/neo-go
 
-if [ -z "$ACC" ]; then
-  ACC=/6000-privnet-blocks.acc.gz
-fi
-
 case $@ in
   "node"*)
   echo "=> Try to restore blocks before running node"
-  if test -f $ACC; then
-    gunzip --stdout /$ACC > /privnet.acc
+  if [ -n "$ACC" -a -f "$ACC" ]; then
+    gunzip --stdout "$ACC" > /privnet.acc
     ${BIN} db restore -p --config-path /config -i /privnet.acc
   fi
     ;;