[mod] reduce enviroment variables in shell scripts to what is needed

Not all settings from the 'brand:' section of the YAML files are needed in the
shell scripts.  This patch reduce the variables in ./utils/brand.env to what is
needed.  The following ('brand:' settings) can be removed from this file:

- ISSUE_URL
- DOCS_URL
- PUBLIC_INSTANCES
- WIKI_URL

Tasks running outside of an *installed instance*, need the following settings
from the YAML configuration:

- GIT_URL            <--> brand.git_url
- GIT_BRANCH         <--> brand.git_branch
- SEARX_URL          <--> server.base_url  (aka PUBLIC_URL)
- SEARX_PORT         <--> server.port
- SEARX_BIND_ADDRESS <--> server.bind_address

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

utils/lxc.sh

@@ -568,7 +568,7 @@ check_connectivity() {
         info_msg "Most often the connectivity is blocked by a docker installation:"
         info_msg "Whenever docker is started (reboot) it sets the iptables policy "
         info_msg "for the FORWARD chain to DROP, see:"
-        info_msg "    ${DOCS_URL}/utils/lxc.sh.html#internet-connectivity-docker"
+        info_msg "    https://searxng.github.io/searxng/utils/lxc.sh.html#internet-connectivity-docker"
         iptables-save | grep ":FORWARD"
     fi
     return $ret_val