fix: a page in the dark color scheme, also one in an iframe, is no longer white while it loads, and test-container.mjs runs the checks with Docker.

package-lock.json

@@ -1,12 +1,12 @@
 {
   "name": "@koszewscy/sk-react-toolkit",
-  "version": "0.7.0",
+  "version": "0.7.1",
   "lockfileVersion": 3,
   "requires": true,
   "packages": {
     "": {
       "name": "@koszewscy/sk-react-toolkit",
-      "version": "0.7.0",
+      "version": "0.7.1",
       "devDependencies": {
         "@emotion/react": "^11.14.0",
         "@emotion/styled": "^11.14.1",