fix: delay upstream DNS resolution to prevent startup failures (#2718)

Use nginx variable pattern (set $upstream + proxy_pass https://$upstream)
to defer DNS resolution until first request instead of at config parse time.

This fixes the occasional 'host not found in upstream' error in CI when
nginx container starts before Docker DNS is fully available.

Fixes #2712

Author: ZackaryShen

travis/libcdb_nginx_cache.conf

@@ -10,20 +10,24 @@ http {
     access_log /dev/stdout cache_st;
     # Use Docker's embedded DNS server to resolve hostnames at runtime
     # instead of at startup.
-    resolver 127.0.0.11 8.8.8.8 ipv6=off;
+    resolver 127.0.0.11 8.8.8.8 ipv6=off valid=30s;
+    # Set default resolver for all upstreams to avoid startup-time DNS resolution
+    # This is a workaround for the occasional 'host not found in upstream' error
+    # in Docker environments where DNS may not be immediately available.
 
     server {
         listen 3000;
         proxy_cache my_cache;
 
         location / {
-            proxy_set_header Host debuginfod.elfutils.org;
+            set $upstream_host "debuginfod.elfutils.org";
+            proxy_set_header Host $upstream_host;
             proxy_ssl_server_name on;
             proxy_cache_revalidate on;
             proxy_cache_key $scheme://$host$uri$is_args$query_string;
             proxy_cache_valid 200 404 12w;
             proxy_cache_use_stale error timeout http_500 http_502 http_503 http_504 http_429;
-            proxy_pass https://debuginfod.elfutils.org/;
+            proxy_pass https://$upstream_host/;
         }
     }
 
@@ -32,14 +36,15 @@ http {
         proxy_cache my_cache;
 
         location / {
-            proxy_set_header Host libc.rip;
+            set $upstream_host "libc.rip";
+            proxy_set_header Host $upstream_host;
             proxy_ssl_server_name on;
             proxy_cache_methods GET HEAD POST;
             proxy_cache_revalidate on;
             proxy_cache_key $scheme://$host$uri$is_args$query_string$request_body;
             proxy_cache_valid 200 404 12w;
             proxy_cache_use_stale error timeout http_500 http_502 http_503 http_504 http_429;
-            proxy_pass https://libc.rip/;
+            proxy_pass https://$upstream_host/;
         }
     }
 
@@ -48,12 +53,13 @@ http {
         proxy_cache my_cache;
 
         location / {
-            proxy_set_header Host archive.ubuntu.com;
+            set $upstream_host "archive.ubuntu.com";
+            proxy_set_header Host $upstream_host;
             proxy_cache_revalidate on;
             proxy_cache_key $scheme://$host$uri$is_args$query_string;
             proxy_cache_valid 200 404 12w;
             proxy_cache_use_stale error timeout http_500 http_502 http_503 http_504 http_429;
-            proxy_pass http://archive.ubuntu.com/;
+            proxy_pass http://$upstream_host/;
         }
     }
 
@@ -62,14 +68,15 @@ http {
         proxy_cache my_cache;
 
         location / {
-            proxy_set_header Host gitlab.com;
+            set $upstream_host "gitlab.com";
+            proxy_set_header Host $upstream_host;
             proxy_ssl_server_name on;
             proxy_cache_revalidate on;
             proxy_cache_key $scheme://$host$uri$is_args$query_string;
             proxy_cache_valid 200 404 12w;
             proxy_ignore_headers Cache-Control Set-Cookie;
             proxy_cache_use_stale error timeout http_500 http_502 http_503 http_504 http_429;
-            proxy_pass https://gitlab.com/;
+            proxy_pass https://$upstream_host/;
         }
     }
 
@@ -78,13 +85,15 @@ http {
         proxy_cache my_cache;
 
         location / {
-            proxy_set_header Host debuginfod.ubuntu.com;
+            set $upstream_host "debuginfod.ubuntu.com";
+            proxy_set_header Host $upstream_host;
             proxy_ssl_server_name on;
             proxy_cache_revalidate on;
             proxy_cache_key $scheme://$host$uri$is_args$query_string;
             proxy_cache_valid 200 404 12w;
             proxy_cache_use_stale error timeout http_500 http_502 http_503 http_504 http_429;
-            proxy_pass https://debuginfod.ubuntu.com/;
+            proxy_pass https://$upstream_host/;
         }
     }
 }
+