entrypoint: default SNIKKET_WEB_SITE_NAME to SNIKKET_SITE_NAME

The documentation only talks about SNIKKET_SITE_NAME,
and users thus do not know that they must
set SNIKKET_WEB_SITE_NAME to make
their site name appear on the portal.

docker/entrypoint.sh

@@ -1,6 +1,9 @@
 #!/bin/sh
 
 export SNIKKET_WEB_DOMAIN="$SNIKKET_DOMAIN"
+if [ -n "${SNIKKET_SITE_NAME:-}" ]; then
+    export SNIKKET_WEB_SITE_NAME="$SNIKKET_SITE_NAME"
+fi
 
 export SNIKKET_TWEAK_PORTAL_INTERNAL_HTTP_INTERFACE="${SNIKKET_TWEAK_PORTAL_INTERNAL_HTTP_INTERFACE-127.0.0.1}"
 export SNIKKET_TWEAK_PORTAL_INTERNAL_HTTP_PORT="${SNIKKET_TWEAK_PORTAL_INTERNAL_HTTP_PORT-5765}"