Docker: add UWSGI_WORKERS and UWSGI_THREAD environment variables (#2992)

* Docker: add UWSGI_WORKERS and UWSGI_THREAD.

UWSGI_WORKERS specifies the number of process.
UWSGI_THREADS specifies the number of threads.

The Docker convention is to specify the whole configuration
through environment variables. While not done in SearXNG, these two
additional variables allows admins to skip uwsgi.ini

In additional, https://github.com/searxng/preview-environments starts Docker
without additional files through searxng-helm-chat.
Each instance consumes 1Go of RAM which is a lot especially when there are a
lot of instances / pull requests.

* [scripts] add environments UWSGI_WORKERS and UWSGI_THREADS

- UWSGI_WORKERS specifies the number of process.
- UWSGI_THREADS specifies the number of threads.

Templates for uwsgi scripts can be tested by::

    UWSGI_WORKERS=8 UWSGI_THREADS=9 \
      ./utils/searxng.sh --cmd\
      eval "echo \"$(cat utils/templates/etc/uwsgi/*/searxng.ini*)\""\
      | grep "workers\|threads"

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

---------

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

utils/templates/etc/uwsgi/apps-archlinux/searxng.ini

@@ -47,6 +47,9 @@ plugin = python
 # default behaviour is for performance reasons.
 enable-threads = true
 
+# Number of workers (usually CPU count)
+workers = ${UWSGI_WORKERS:-%k}
+threads = ${UWSGI_THREADS:-4}
 
 # plugin: python
 # --------------

utils/templates/etc/uwsgi/apps-archlinux/searxng.ini:socket

@@ -47,6 +47,9 @@ plugin = python
 # default behaviour is for performance reasons.
 enable-threads = true
 
+# Number of workers (usually CPU count)
+workers = ${UWSGI_WORKERS:-%k}
+threads = ${UWSGI_THREADS:-4}
 
 # plugin: python
 # --------------