Write and retrieve files in Heroku and Dockerized fastAPI project - docker

Question

I'm struggling with this issue:
I've a fastAPI app with a couple of routes that are called by other services and I've to put up a testing environment for the customers to try out the features.
I've containerized the whole api project and it works very well.
Then I've published on Heroku via container stack the whole thing and now it doesn't save the files in the /response directory.
My Dockerfile is:
FROM python:3.9-slim
COPY ./src /app/src
RUN mkdir /app/responses
COPY ./requirements.txt /app
COPY ./start.sh /app
COPY ./templates /app/templates
COPY ./static /app/static
WORKDIR /app
RUN pip install -r requirements.txt
RUN chmod a+x ./start.sh
#EXPOSE 8000
CMD ["./start.sh"]
My fastAPI app has mounted the static file directory (which is not working either) and the responses directory as follows:
from fastapi import FastAPI, File, UploadFile, Request, Form
from fastapi.responses import HTMLResponse, FileResponse
from fastapi.staticfiles import StaticFiles
from fastapi.templating import Jinja2Templates
app.mount("/static", StaticFiles(directory="static"), name="static")
app.mount("/responses", StaticFiles(directory="responses"), name="responses")
templates = Jinja2Templates(directory="templates")
And then I try to write the files with this code:
filename = fiscalCode + "/" + fiscalCode +"_" +timestamp+".html"
filepath = os.path.join(RESPONSE_FOLDER, filename)
print(filepath)
if not os.path.exists(filepath):
os.makedirs(os.path.dirname(filepath), exist_ok=True)
with open(filepath, "w+") as f:
f.write(file.file.read().decode("utf-8"))
I know that Heroku filesystem is ephemeral but this is not an issue: the file must be available only for a couple of minutes for testing purposes.
I'm stuck bcs on other applications deployed via Heroku and Procfile (so without the Docker shenanigans) I had no issues at all in writing and retrieving files.
Thanks for any idea.

Answer 1

I found that, even in Heroku remote shell, I can't see the files created, I think because of the ephemeral file system.
Nevertheless all the functions are working fine, and transitional files are stored somewhere in the hyperuranium.
So if you don't find your actual files, they are written on (I suppose) a caching memory or something like that.