Hi @ignacioct, welcome to the community!! 
That welcome message is displayed when there does not exist a ~/.streamlit/credentials.toml file with the following content:
[general]
email=""
You can either create the above file (.streamlit/credentials.toml) within your app directory and copy its content to the container image in your Dockerfile or create this file using RUN commands on the following:
mkdir -p ~/.streamlit/
echo "[general]" > ~/.streamlit/credentials.toml
echo "email = \"\"" >> ~/.streamlit/credentials.toml
I would suggest the former approach to reduce the number of layers and thereby reduce the final image size.
Best, 
Snehan