Docker: show welcome page on port 80 when entrypoint.d is empty.

The entrypoint script now performs a default configuration when no useful
files are found in /docker-entrypoint.d/

The default configuration serves a welcome page in response to all
requests, using Markdown unless text/html is sent in the Accept header.

This provides a useful 'hello world' experience when running a Unit
container for the first time.

pkg/docker/template.Dockerfile

@@ -75,9 +75,10 @@ RUN set -ex \
     && ln -sf /dev/stdout /var/log/unit.log
 
 COPY docker-entrypoint.sh /usr/local/bin/
+COPY welcome.* /usr/share/unit/welcome/
 
 STOPSIGNAL SIGTERM
 
 ENTRYPOINT ["/usr/local/bin/docker-entrypoint.sh"]
-
+EXPOSE 80
 CMD ["unitd", "--no-daemon", "--control", "unix:/var/run/control.unit.sock"]