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/welcome.md

@@ -0,0 +1,29 @@
+Welcome to NGINX Unit
+=====================
+
+Congratulations! NGINX Unit is installed and running.
+
+Useful Links
+------------
+
+ * https://unit.nginx.org/
+   - Get started with the 'Configuration' docs, starting with the 'Quick Start' guide.
+
+ * https://unit.nginx.org/howto/docker/
+   - Guidance for running Unit in a container and tips for containerized applications.
+
+ * https://github.com/nginx/unit
+   - See our GitHub repo to browse the code, contribute, or seek help from the community.
+
+Current Configuration
+---------------------
+Unit's control API is currently listening for configuration changes on the Unix socket at
+`/var/run/control.unit.sock` inside the container.
+
+Read the current configuration with
+```
+docker exec -ti <containerID> curl --unix-socket /var/run/control.unit.sock http://localhost/config
+```
+
+---
+NGINX Unit - the universal web app server