fix: can't check from inside docker if host file exists.

This check can't be done, but the default standard location where it
should have been bind mounted should be testable. This is done on the
charm's end.

bin/compose-core

@@ -818,7 +818,7 @@ _dcmd() {
 
     debug "Db> $command $@"
 
-    if [ -f "$HOST_DB_PASSFILE" -a "$CLIENT_DB_PASSFILE" ]; then
+    if [ "$HOST_DB_PASSFILE" -a -f "$LOCAL_DB_PASSFILE" -a "$CLIENT_DB_PASSFILE" ]; then
         verb "Found and using '$HOST_DB_PASSFILE' as '$CLIENT_DB_PASSFILE'."
         docker_opts=("${db_docker_opts[@]}" "-v" "$HOST_DB_PASSFILE:$CLIENT_DB_PASSFILE")
     else