docker: improve image layout and backend-only/frontend-only images (#2071)

* docker: improve image layout and backend-only image

* add tests

* add tests

* add frontend

* add frontend

* label

* fix

docker/frontend/start-nginx.sh

@@ -0,0 +1,16 @@
+#!/bin/sh
+set -e
+trap 'kill -TERM $PID' TERM INT
+if [ -z "$BACKEND_URL" ]; then
+  BACKEND_URL="$1"
+fi
+if [ -z "$BACKEND_URL" ]; then
+  echo "BACKEND_URL must be set as an environment variable or as first parameter. (e.g. http://localhost:7860)"
+  exit 1
+fi
+sed -i "s|__BACKEND_URL__|$BACKEND_URL|g" /etc/nginx/conf.d/default.conf
+cat /etc/nginx/conf.d/default.conf
+
+
+# Start nginx
+exec nginx -g 'daemon off;'