Also avoid installing recommended packages if possible to silightly
reduce the Docker image size.

Change-Id: I7a3ea33f6501cf063b1aa0324b5928f96241d048
Reviewed-on: https://go-review.googlesource.com/105077
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>