feat: enable API_URL in Dockerfile

fix tranga.conf not redirecting /api/ requests

nginx/templates/default.conf.template

@@ -0,0 +1,29 @@
+server {
+    listen       80 default_server;
+    listen  [::]:80 default_server;
+
+    http2  on;
+
+    server_name  _;
+
+    access_log  /dev/stdout  main;
+    error_log  /dev/stderr;
+
+    location /api/ {
+        proxy_pass_request_headers on;
+        rewrite ^/api/(.*)$ /$1 break;
+        proxy_pass $API_URL;
+    }
+
+    location / {
+        root   /usr/share/nginx/html;
+        index  index.html index.htm;
+    }
+
+    # redirect server error pages to the static page /50x.html
+    #
+    error_page   500 502 503 504  /50x.html;
+    location = /50x.html {
+        root   /usr/share/nginx/html;
+    }
+}