docker-compose set environment per-service profile - docker-compose

Question

I recently discovered docker-compose profiles, which seem great for allowing optional local resources for testing
However, it's not clear if it's possible to provide a container with a different environment depending on the profile; what is a sensible way (if any) to switch environmental vars by-service profile?
Perhaps
using extends (which appears deprecated, but may work for me anyways Extend service in docker-compose 3)
the profile value is or can be made available to the container so it can switch internally
this was never intended or considered in the design (probe local connection on startup, volume mounting tricks..)
Specifically, I'm trying to prefer an address and some keys via env var under some testing profile, but prefer a .env file otherwise.
Normal structure
services:
webapp:
...
env_file:
- .env
Structure with test profile
services:
db-service:
image: db-image
profiles: ["test"]
...
webapp:
...
environment:
- DATABASE_HOST=db-service:1234

Answer 1

I can say with certainty that this was never an intended use case for profiles :)
docker-compose has no native way to pass the current profile down to a service. As a workaround you could pass the COMPOSE_PROFILES environment variable to the container. But this does not work when specifying the profiles with the --profiles flag on the command line.
Also you had to manually handle having multiple active profiles corretly.
The best solution for your specific issue would be to have different services for each profile:
services:
webapp-prod:
profiles: ["prod"]
#...
env_file:
- .env
db-service:
image: db-image
profiles: ["test"]
#...
webapp-test:
profiles: ["test"]
#...
environment:
- DATABASE_HOST=db-service:1234
This only has the downside of different service names for "the same" service with different configurations and they both need assigned profile(s) so none of them will start by default, i.e. with every profile.
Also it has some duplicate code for the two service definitions. If you want to share the definition in the file you could use yaml anchors and aliases:
services:
webapp-prod: &webapp
profiles: ["prod"]
#...
env_file:
- .env
webapp-test:
<<: *webapp
profiles: ["test"]
environment:
- DATABASE_HOST=db-service:1234
db-service:
image: db-image
profiles: ["test"]
#...
Another alternative could be using multiple compose files:
# docker-compose.yml
services:
webapp:
#...
env_file:
- .env
# docker-compose.test.yml
services:
db-service:
image: db-image
#...
webapp:
environment:
- DATABASE_HOST=db-service:1234
This way you can start the production service normally and the instances by passing and merging the compose files:
docker-compose up # start the production version
docker-compose -f docker-compose.yml -f docker-compose.test.yml # start the test version

Answer 2

Arcan's answers has a lot of good ideas.
I think another solution is to just pass a variable next to your --profile tag on your docker commands. You can then for instance set an -e TESTING=.env.testing in your docker-compose command and use env_file:${TESTING:-.env.default} in your file. This allows you to have a default env file added on any none profile actions and runs the given file when needed.
Since I have a slightly different setup I am adding a single variable to a container in my docker-compose so I did not test if it works on the env-file: attribute but I think it should work.