.dockerignore: exclude devtools/config and e2e directories

Change-Id: I6e3d1df9ec55e3bd3ec3b1f3383c9a218dc8b868
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/292692
Reviewed-by: Julie Qiu <julie@golang.org>
Trust: Jamal Carvalho <jamal@golang.org>

.dockerignore

@@ -6,3 +6,7 @@ node_modules
 private
 # ...except for the config directory.
 !private/config
+# Exclude e2e tests.
+e2e
+# Exclude devtools config.
+devtools/config