WR340583, Ensure docker base image is up to date

Building a docker image from a stale base image can give unexpected
results. Add build.builder.sh script to make is easy to build the
image correctely.

README.md

@@ -14,6 +14,9 @@ The first command only builds English. The second builds all translations
 (and takes longer).
 
 ```
+  # build/update the dockerized manual builder
+  manual-build/build-builder.sh
+
   manual-build/build-manual.sh TRANSLATIONS=en html
   manual-build/build-manual.sh html
 ```
@@ -41,10 +44,12 @@ you continue.
 If docker is available you can build the docbuilder image on Linux with:
 
 ```
-cd manual-build
-docker build --build-arg IMAGE_UID=`id -u` --build-arg IMAGE_GID=`id -g` --tag mahara/docbuilder .
+./manual-build/build-builder.sh
 ```
 
+Note: this command should be run periodically to freshen up (patch) the builder
+image.
+
 The `mahara/docbuilder` image is based on a debian docker image with additional
 packages that are required for building the docs installed. Using a docker image
 for this makes it easy to get going with the same build environment as used by

build-builder.sh

+#!/bin/bash
+# Builds and/or refreshes the Mahara docbuilder image.
+
+BASE_DIR=$(dirname $0)
+
+cd ${BASE_DIR}
+docker pull debian:stable
+docker build --build-arg IMAGE_UID=`id -u` --build-arg IMAGE_GID=`id -g` --tag mahara/docbuilder .