Using git repo as build context inside stack gives error "Could not locate dockerfile"

Hi guys,

simple question. When I edit my stack and add a git repository as build context to a service, like this:

services:
  web:
    build: https://github.com/buhrmi/somerepo

i get the following error:

Capture

Does anybody know what might be wrong or how I can fix it?

best,
buhrmi