apache: reverse proxy, set `ProxyPreserveHost On`

related discussions:

- https://github.com/asciimoo/searx/issues/1822
- https://github.com/asciimoo/searx/issues/1819#issuecomment-580400259

Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>

utils/templates/etc/apache2/sites-available/searx.conf:filtron

@@ -1,15 +1,23 @@
 # -*- coding: utf-8; mode: apache -*-
 
+ProxyPreserveHost On
+
 <Location ${FILTRON_URL_PATH} >
 
     Require all granted
-    Allow from all
-
     Order deny,allow
     Deny from all
     #Allow from fd00::/8 192.168.0.0/16 fe80::/10 127.0.0.0/8 ::1
+    Allow from all
 
-    ProxyPass "http://127.0.0.1:4004"
+    ProxyPass http://${FILTRON_LISTEN}
-    RequestHeader set "X-Script-Name" "${FILTRON_URL_PATH}"
+    RequestHeader set X-Script-Name ${FILTRON_URL_PATH}
+
+    # In Apache it seems, that setting HTTP_HOST header direct here does have no
+    # effect.  I needed to set 'ProxyPreserveHost On' (see above).  HTTP_HOST is
+    # needed by searx to render correct *Search URL* in the *Link* box and
+    # *saved preference*.
+
+    # RequestHeader set Host ${PUBLIC_HOST}
 
 </Location>