[enh] container: custom certificates (#5238)

Let container instance administrators to add custom certificates:

  https://docs.searxng.org/admin/installation-docker.html#custom-certificates

Closes https://github.com/searxng/searxng/issues/5206

container/base-builder.yml

@@ -16,6 +16,7 @@ work-dir: /usr/local/searxng/
 
 environment:
   PATH: /usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
+  SSL_CERT_DIR: /etc/ssl/certs
   SSL_CERT_FILE: /etc/ssl/certs/ca-certificates.crt
   HISTFILE: /dev/null