gitignore: sync with master branch

Signed-off-by: Gyuho Lee <gyuhox@gmail.com>

.gitignore

@@ -1,16 +1,22 @@
 /agent-*
 /coverage
+/covdir
 /gopath
 /gopath.proto
 /go-bindata
+/release
 /machine*
 /bin
+.Dockerfile-test
 .vagrant
 *.etcd
+*.log
 /etcd
 *.swp
 /hack/insta-discovery/.env
 *.test
 tools/functional-tester/docker/bin
+hack/scripts-dev/docker-dns/.Dockerfile
+hack/scripts-dev/docker-dns-srv/.Dockerfile
 hack/tls-setup/certs
 .idea