fix healthcheck for flask

Author: lionello

samples/flask/compose.yaml

@@ -14,4 +14,4 @@ services:
         target: 5000
         published: 5000
     healthcheck:
-      test: ["CMD", "curl", "-f", "http://localhost:5000/"]
+      test: ["CMD", "python3", "-c", "import sys, urllib.request; urllib.request.urlopen(sys.argv[1]).read()", "http://localhost:5000/"]