dockerignore: ignore .git (#1723)

also ignore build/ and release/ as they may be left over from releasing
coredns.

Sending build context to Docker daemon  490.9MB ->
 Sending build context to Docker daemon  98.82kB

.dockerignore

+.git/*
 core/*
 coremain/*
 hooks/*
@@ -7,3 +8,5 @@ plugin/*
 request/*
 test/*
 vendor/*
+build/*
+release/*