add docker in Makefile.docker (#4864)

This adds the (older) docker stuff back in a new Makefile.docker. Add
.dreck alias to be able to release the docker stuff via a github issue.

See #4858

Signed-off-by: Miek Gieben <miek@miek.nl>

.dreck.yaml

@@ -9,3 +9,5 @@ aliases:
     /wai -> /label works as intended
   - |
     /release (.*) -> /exec /opt/bin/release-coredns $1
+  - |
+    /docker (.*) -> /exec /opt/bin/docker-coredns $1