How to get updates(response) from a REST API implemented using Flask? Is there any workaround if it is not possible?
An REST-API is synchronous, but your processing does not need to be.
You should probably change your architecture so that your HTTP-POST just creates all necessary jobs and returns ids the user can check on later.
When the user wants to get an update, he has to ask your api using the id to get the status.
Obviously, all of your containers need to support that pattern - e.g. they send asynchronous updates about the status of the processing. In my experience, rest api calls are not the best pattern for this, use something asynchronous to report the status, e.g. RabbitMq / kafka / nats.
Your All in One container / or a new dedicated aggregator can then aggreagte such updates and return the status if somebody asks.
I am working on an OCR application. I use docker for my application. The application consists of 4 containers.

Converter (Converts PDF to the image)
Cleaner (Cleans the image)
Transcriber (Converts image to text)
All in One

The AllinOne container integrates all the other containers through API calls. The APIs are implemented using Flask. The AllinOne container is exposed to the server.  The workflow is as follows,
Postman (User Request) -> AllinOne -> Convertor -> AllinOne -> Cleaner -> Allinone -> Transcriber -> Allinone -> Postman (response)
Now, I am trying to add a feature to my application called the Bulk Upload, where users will be able to upload a .zip file. Since bulk uploads take a longer time to process, I need intermediate results about the status of the process. I have a file_id allocated for each file that is being uploaded. I need to return the file_id to the user so that they'll be able to check the status of processing for their file. But, the processing has to take place in the background.
If I return the file_id the processing would stop, or else the file_id would be returned only after processing of the file, which is useless. Should I implement two API calls? One for uploading the file and returns the file_id, and the other to process the file and return the result. Is there a way to send frequent updates as a response from an API? Can it be done better? Feel free to suggest any change in the architecture.