use build arg for wiki version & for container tag

There is a new version of wiki available.  Want to tag this version
and enable building and tagging the next version.

Dockerfile

@@ -5,7 +5,8 @@ RUN useradd --create-home app \
  && apt-get install -y --no-install-recommends \
     jq
 WORKDIR /home/app
-RUN su app -c "npm install -g --prefix . wiki@0.11.4"
+ARG VERSION=0.11.4
+RUN su app -c "npm install -g --prefix . wiki@$VERSION"
 RUN su app -c "mkdir .wiki"
 COPY configure-and-launch-wiki set-owner-name ./
 RUN chown app configure-and-launch-wiki set-owner-name

README.md

@@ -49,3 +49,11 @@ docker run -p 3000:3000 -it --rm \
 
 Open http://this.localtest.me:3000 in one tab.
 Open http://that.localtest.me:3000 in another.
+
+# Development
+
+``` bash
+VERSION=0.11.4
+docker build --build-arg VERSION=$VERSION -t dobbs/farm:$VERSION .
+git tag $VERSION
+```