Define $is_mobile / $is_desktop based on simple UA matching

xh.conf

@@ -19,6 +19,22 @@ map $sent_http_content_type $expires {
 # Increase client max body size to allow for larger uploads
 client_max_body_size 20m;
 
+# Set $is_desktop and $is_mobile variables for simple UA-based sniffing.
+# This was snagged from https://gist.github.com/perusio/1326701 and is not guaranteed by any means
+# to be perfect or exhaustive. Apps should use with care - we can tune if we have real-world cases
+# where we find this is not working as desired.
+map $http_user_agent $is_desktop {
+    default 0;
+    ~*linux.*android|windows\s+(?:ce|phone) 0; # exceptions to the rule
+    ~*spider|crawl|slurp|bot 1; # bots
+    ~*windows|linux|os\s+x\s*[\d\._]+|solaris|bsd 1; # OSes
+}
+
+map $is_desktop $is_mobile {
+    1 0;
+    0 1;
+}
+
 # App-level Dockerfiles must copy in an appropriate conf with server directives such as the below.
 
 #server {