[fix] utils/build_env.py and add documentation

modified   docs/admin/engines/settings.rst
  - Fix documentation and add section 'brand'.
  - Add remarks about **buildenv** variables.
  - Add remarks about settings from environment variables $SEARX_DEBUG,
    $SEARX_PORT, $SEARX_BIND_ADDRESS and $SEARX_SECRET

modified   docs/admin/installation-searx.rst & docs/build-templates/searx.rst
   Fix template location /templates/etc/searx/settings.yml

modified   docs/dev/makefile.rst
  Add description of the 'make buildenv' target and describe
  - we have all SearXNG setups are centralized in the settings.yml file
  - why some tasks need a utils/brand.env (aka instance's buildenv)

modified   manage
  Settings file from repository's working tree are used by default and
  ask user if a /etc/searx/settings.yml file exists.

modified   searx/settings.yml
  Add comments about when it is needed to run 'make buildenv'

modified   searx/settings_defaults.py
  Default for server:port is taken from enviroment variable SEARX_PORT.

modified   utils/build_env.py
  - Some defaults in the settings.yml are taken from the environment,
    e.g. SEARX_BIND_ADDRESS (searx.settings_defaults.SHEMA).  When the
    'brand.env' file is created these enviroment variables should be
    unset first.
  - The CONTACT_URL enviroment is not needed in the utils/brand.env

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

docs/dev/makefile.rst

@@ -81,6 +81,30 @@ the check fails if you edit the requirements listed in
 If you think, something goes wrong with your ./local environment or you change
 the :origin:`setup.py` file, you have to call :ref:`make clean`.
 
+.. _make buildenv:
+
+``make buildenv``
+=================
+
+Rebuild instance's environment with the modified settings from the
+:ref:`settings global brand` and :ref:`settings global server` section of your
+:ref:`settings.yml <settings location>`.
+
+We have all SearXNG setups are centralized in the :ref:`settings.yml` file.
+This setup is available as long we are in a *installed instance*.  E.g. the
+*installed instance* on the server or the *installed developer instance* at
+``./local`` (the later one is created by a :ref:`make install <make
+install>` or :ref:`make run <make run>`).
+
+Tasks running outside of an *installed instance*, especially those tasks and
+scripts running at (pre-) installation time do not have access to the SearXNG
+setup (from a *installed instance*).  Those tasks need a *build environment*.
+
+The ``make buildenv`` target will update the *build environment* in:
+
+- :origin:`utils/brand.env`
+
+
 .. _make run:
 
 ``make run``