Hi!
This is actually possible to do.
First, create file named requirements-plugins.txt in the same directory that contains requirements.txt (it should be misago). Inside this file put this line:
django-storages
Next, create settings_override.py file in same directory that contains settings.py (it should be misago/misagodocker). Fill its contents like this:
DEFAULT_FILE_STORAGE = "storages.backends.s3boto3.S3Boto3Storage"
AWS_ACCESS_KEY_ID = "YOUR AWS ACCESS KEY ID HERE"
AWS_SECRET_ACCESS_KEY = "YOUR AWS SECRET ACCESS KEY HERE"
AWS_STORAGE_BUCKET_NAME = "YOUR BUCKET NAME"
AWS_S3_REGION_NAME = "YOUR REGION NAME"
Remember to fill in valid values to AWS_ settings!
Then, run ./appctl rebuild to rebuild your Misago site. After rebuild is complete, Misago should use S3 for uploads storage.