Can't launch setup.py in Docker image - module named 'flask'

I'm trying to debug why the server settings are not applied from the
/pgadmin4/servers.json file in Docker container (with clean
/var/lib/pgadmin directory), via manually launch this process:

python3 /pgadmin4/setup.py --load-servers /pgadmin4/servers.json

but got the error:
---
Traceback (most recent call last):
File "/pgadmin4/setup.py", line 33, in <module>
from pgadmin.model import db, User, Version, ServerGroup, Server, \
File "/pgadmin4/pgadmin/__init__.py", line 23, in <module>
from flask import Flask, abort, request, current_app, session, url_for
ModuleNotFoundError: No module named 'flask'
---

So, if it can't be launched manually, it must fail on automatic servers
import process too?

Is this a missing package in Docker image, or some misconfiguration?

And is there any other ways to debug the applying servers.json process?

--
Best regards,
Alexey Murz Korepov.
E-mail: murznn(at)gmail(dot)com
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@MurzNN