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>

util/docker/doc.coreboot.org/Dockerfile

@@ -1,5 +1,22 @@
-FROM debian:stable
-RUN apt-get update && apt-get install -y make python-sphinx python-recommonmark python-sphinx-rtd-theme git && apt-get clean
-USER nobody
+FROM alpine:3.8
+
+COPY makeSphinx.sh /makeSphinx.sh
+
+RUN apk add --no-cache python3 make bash git \
+ && pip3 install --upgrade --no-cache-dir pip \
+ && pip3 install --no-cache-dir  \
+    sphinx===1.7.7 \
+    sphinx_rtd_theme===0.4.1 \
+    recommonmark===0.4.0 \
+    sphinx_autobuild===0.7.1 \
+ && chmod 755 /makeSphinx.sh
+
 VOLUME /data-in /data-out
-ENTRYPOINT bash -c "cd /data-in/Documentation && make sphinx BUILDDIR=/tmp/build && rm -rf /data-out/* && mv /tmp/build/html/* /data-out/"
+
+# For Sphinx-autobuild
+# Port 8000 - HTTP server
+# Port 35729 - websockets connection to allow automatic browser reloads after each build
+EXPOSE 8000 35729
+
+ENTRYPOINT ["/bin/bash", "/makeSphinx.sh"]
+CMD []