[Docker file] Fix wrong version string in docker images (RSS-Bridge#2497)

do not fully ignore git directory when building docker images
".git/HEAD" and ".git/refs/heads/*" are required by "getVersion()" in "lib/Configuration.php" to build the version string

Author: User123698745

.dockerignore

@@ -1,4 +1,6 @@
 .git
+!.git/HEAD
+!.git/refs/heads/*
 .gitattributes
 .github/*
 .travis.yml