In portainer I created an image, using the web editor to enter the Dockerfile commands.
But I see no option to modify that image, there is no edit options.
Am I supposed to have the Dockerfile stored somewhere else, then paste it into portainer every time I've edited it?
Portainer do not allow you to edit an image from a Dockerfile as it does not store the Dockerfile.
I'd recommend to version your Dockerfile in a CVS, this would allow to version any changes to your Dockerfile, and then update your image via the upload method inside Portainer when needed.
