@Arek Thanks for reaching out. If you need to use Docker Container for your function app, you may consider upgrading to a Pay-As-You-Go subscription and make sure you create a function app in a Premium or Dedicated plan.
Note that as docker container is only supported for functions running in Linux Premium and Dedicated plan (Deployment technologies in Azure Functions | Microsoft Learn).
do let me know incase of further queries, I would be happy to assist you. Please do not forget to "Accept the answer” and “up-vote” wherever the information provided helps you, this can be beneficial to other community members.