[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.yml

@@ -3,6 +3,7 @@ contents:
     - https://dl-cdn.alpinelinux.org/alpine/edge/main
   packages:
     - alpine-baselayout
+    - ca-certificates
     - ca-certificates-bundle
     - musl-locales
     - musl-locales-lang
@@ -27,6 +28,7 @@ accounts:
 
 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
   CONFIG_PATH: /etc/searxng