dockerignore: ignore .git (#1723)
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
ii/coredns,jremond/coredns,coredns/coredns,chrisohaver/coredns,johnbelamaric/coredns,miekg/coredns,yongtang/coredns,jremond/coredns,johnbelamaric/coredns,ii/coredns,miekg/coredns