Docker Swarm with image versions externalized to .env file
The yaml parser in docker stack deploy doesn't have all the same features of that in docker-compose. However, you can use docker-compose config to output a yaml file after it's done all the variable substitutions, extending other files, and merging multiple files together. This effectively turns docker-compose into a preprocessor.
The answer is quite simple: it's not a bug, nor a feature. .env is not currently supported by docker stack.You must source manually the .env running export $(cat .env) before running docker stack ...
There is an issue discussing this needs in the Docker Github.https://github.com/docker/docker.github.io/issues/3654and another one discussing the problem and solution: https://github.com/moby/moby/issues/29133#issuecomment-285980447
As already mentioned the .env isn't currently supported by the docker stack.So the alternative way is to clearly specify the environment file using by env_file, e.g.:
version: '3.3'services: foo-service: image: foo-image env_file: - .env environment: - SOME_ENV=qwerty