how to use environment variable in docker-compose child Dockerfile FROM clause - docker

Question

I'm using docker-compose to build some dockerfiles. I'm using an environment variable $IMAGE_TAG to tag images
game-client:
image: 09809809.dkr.ecr.us-east-1.amazonaws.com/crashgiants-web:${IMAGE_TAG}
build:
context: ..
dockerfile: devops/client.Dockerfile
args:
- IMAGE_TAG=${IMAGE_TAG}
- SIGNALHUB_HOST=crashgiants.io
- SOCKETIO_HOST=crashgiants.io
- TURNSERVER_HOST=crashgiants.io
- ASSET_HOST=crashgiants.io
ports:
- "80:80"
I have a dockerfile client.Dockerfile with a FROM clause like:
FROM 0809809708.dkr.ecr.us-east-1.amazonaws.com/crashgiants:$IMAGE_TAG as game-base
I can get the environment variable to the docker-compose ok but not sure how to use it within the child client.Dockerfile
I am invoking the compose file with: IMAGE_TAG=local_tester docker-compose -f ./devops/docker-compose.yml build
This currently throws an error
Building game-client
Traceback (most recent call last):
File "compose/cli/main.py", line 67, in main
File "compose/cli/main.py", line 126, in perform_command
File "compose/cli/main.py", line 302, in build
File "compose/project.py", line 468, in build
File "compose/project.py", line 450, in build_service
File "compose/service.py", line 1147, in build
compose.service.BuildError: (<Service: game-client>, {'message': 'No build stage in current context'})
During handling of the above exception, another exception occurred:
Traceback (most recent call last):
File "docker-compose", line 3, in <module>
File "compose/cli/main.py", line 78, in main
TypeError: can only concatenate str (not "dict") to str
[95080] Failed to execute script docker-compose
something about can only concatenate str (not "dict") to str

Answer 1

Modify your dockerfile as follows:
ARG IMAGE_TAG=latest
FROM 0809809708.dkr.ecr.us-east-1.amazonaws.com/crashgiants:$IMAGE_TAG as game-base
ARG IMAGE_TAG
This will expose the IMAGE_TAG as argument to the image. The second ARG IMAGE_TAG is if you want to use the argument inside the image build, since the arguments before FROM will be in a different context.