util/docker/doc.coreboot.org/Dockerfile: Use alpine:3.8, Sphinx 1.7

With Alpine base, use pip to install Sphinx 1.7 and Sphinx-autobuild
Alpine, a 4.5MB base, is used over Debian Stable, 101MB, to cut down the
total size of the docker image.

Change-Id: I53f246206458b1de34cd7f3a42481b91ca285ff0
Signed-off-by: Tom Hiller <thrilleratplay@gmail.com>
Reviewed-on: https://review.coreboot.org/28211
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Patrick Georgi <pgeorgi@google.com>

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