[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>

utils/build_env.py

@@ -9,15 +9,12 @@ from os.path import realpath, dirname, join, sep, abspath
 
 repo_root = realpath(dirname(realpath(__file__)) + sep + '..')
 sys.path.insert(0, repo_root)
-os.environ['SEARX_SETTINGS_PATH'] = abspath(dirname(__file__) + '/settings.yml')
 
 # Under the assumption that a brand is always a fork assure that the settings
 # file from reposetorie's working tree is used to generate the build_env, not
 # from /etc/searx/settings.yml.
 os.environ['SEARX_SETTINGS_PATH'] = abspath(dirname(__file__) + sep + 'settings.yml')
 
-from searx import get_setting
-
 def _env(*arg, **kwargs):
     val = get_setting(*arg, **kwargs)
     if val is True:
@@ -27,19 +24,35 @@ def _env(*arg, **kwargs):
     return val
 
 name_val = [
-    ('SEARX_URL'              , _env('server.base_url','')),
-    ('GIT_URL'                , _env('brand.git_url', '')),
-    ('GIT_BRANCH'             , _env('brand.git_branch', '')),
-    ('ISSUE_URL'              , _env('brand.issue_url', '')),
-    ('DOCS_URL'               , _env('brand.docs_url', '')),
-    ('PUBLIC_INSTANCES'       , _env('brand.public_instances', '')),
-    ('CONTACT_URL'            , _env('general.contact_url', '')),
-    ('WIKI_URL'               , _env('brand.wiki_url', '')),
+    ('SEARX_URL'              , 'server.base_url'),
+    ('GIT_URL'                , 'brand.git_url'),
+    ('GIT_BRANCH'             , 'brand.git_branch'),
+    ('ISSUE_URL'              , 'brand.issue_url'),
+    ('DOCS_URL'               , 'brand.docs_url'),
+    ('PUBLIC_INSTANCES'       , 'brand.public_instances'),
+    ('WIKI_URL'               , 'brand.wiki_url'),
 ]
 
 brand_env = 'utils' + sep + 'brand.env'
 
-print('build %s' % brand_env)
+# Some defaults in the settings.yml are taken from the environment,
+# e.g. SEARX_BIND_ADDRESS (:py:obj:`searx.settings_defaults.SHEMA`).  When the
+# 'brand.env' file is created these enviroment variables should be unset first::
+
+_unset = object()
+for name, option in name_val:
+    if not os.environ.get(name, _unset) is _unset:
+        del os.environ[name]
+
+# After the variables are unset in the environ, we can import settings
+# (get_setting) from searx module.
+
+from searx import get_setting
+
+print('build %s (settings from: %s)' % (brand_env, os.environ['SEARX_SETTINGS_PATH']))
+sys.path.insert(0, repo_root)
+from searx import settings
+
 with open(repo_root + sep + brand_env, 'w', encoding='utf-8') as f:
-    for name, val in name_val:
-        print("export %s='%s'" % (name, val), file=f)
+    for name, option in name_val:
+        print("export %s='%s'" % (name, _env(option)), file=f)