.dockerignore: files that docker should exclude

We'll want this to build docker images for our binaries. Without a
.dockerignore file, the build context is 153MB (docker must upload
that all to the daemon). This file reduces that to under 11MB.

Change-Id: I08973850e5a55e301239cf97d9b2ba9c43a5a590
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/250018
Reviewed-by: Julie Qiu <julie@golang.org>

.dockerignore

@@ -0,0 +1,6 @@
+# Exclude all git directories.
+**/.git
+# Exclude the private sub-repo...
+private
+# ...except for the config directory.
+!private/config