ModuleNotFoundError: No module named ‘discord_webhook’

So I have a docker container which is trying to use a python module called discord_webhook, to integrate my web service to discord, however when I start the container, it instantly errors out saying

houdini_blizzard_1  |   File "/usr/src/houdini/houdini/plugins/discordwebhook/__init__.py", line 3, in <module>
houdini_blizzard_1  |     from discord_webhook import DiscordWebhook, DiscordEmbed
houdini_blizzard_1  | ModuleNotFoundError: No module named 'discord_webhook'

I used docker exec -it {container-id} sh to connect to the container, and ran pip install discord_webhook. It installed fine with no errors. But I then I added discord_webhook module back to the __init__.py of the plugin, then restarted the docker container, but it still gave the same error. When I ls in /usr/local/lib/python3.9/site-packages/discord_webhook there are these files/folders

__init__.py  __main__.py  __pycache__  webhook.py

That should mean that discord_webhook is installed properly. So why is python saying the module isn’t found?

Source: Docker Questions