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.json

@@ -0,0 +1,25 @@
+{
+    "listeners": {
+        "*:80": {
+            "pass": "routes"
+        }
+    },
+
+    "routes": [
+        {
+            "match": {
+                "headers": {
+                    "accept": "*text/html*"
+                }
+            },
+            "action": {
+                "share": "/usr/share/unit/welcome/welcome.html"
+            }
+        },
+        {
+            "action": {
+                "share": "/usr/share/unit/welcome/welcome.md"
+            }
+        }
+    ]
+}