Setting environment inside a docker container from secrets

In a docker swarm, I need to setup an environment variable to the value of a docker secret. I followed the answer here , but it does not work for me.

To try various options, I created a secret called testpwd and made a very simple compose file for the docker stack, as follows:

services:
  server:
    image: bash
    deploy:
      replicas: 1
    entrypoint: ["sh", "-c", "cat /run/secrets/testpwd; TESTSEC=`cat /run/secrets/testpwd`; export TESTSEC; echo TESTSEC; echo $TESTSEC; ping stackoverflow.com"]
    secrets:
      - testpwd

secrets:
  testpwd:
    external: true

A log from the container launched by this stack shows:

pwd1234

TESTSEC

PING stackoverflow.com (104.18.32.7): 56 data bytes 64 bytes from 104.18.32.7: seq=0 ttl=56 time=2.548 ms

The content inside /run/secrets/testpwd is pwd1234 but TESTSPEC is neither created as an environment variable for this shell nor it is exported.

I have tried multiple variants of the entrypoint, such as:

entrypoint: ["sh", "-c", "export TESTSEC=$$(cat /run/secrets/testpwd); ping stackoverflow.com"]
entrypoint: ["sh", "-c", "export TESTSEC=$$(cat /run/secrets/testpwd) && ping stackoverflow.com"]

and others mentioned in the link at the begining.

But nothing works. When I get inside the container with "docker exec -it cont_id sh", I can see that the ping is running and see the value of the secret in /run/secrets/testpwd, but there is no TESTSEC environment variable.

Just in case, in the host machine:

$docker --version
Docker version 26.1.0, build 9714adc

$docker compose version
Docker Compose version v2.26.1

$lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description:    Ubuntu 22.04.4 LTS
Release:        22.04
Codename:       jammy

stack.yaml

services:
  server:
    image: bash
    deploy:
      mode: replicated-job
      replicas: 1
    entrypoint: 
    - sh
    - -c
    - | 
      export TESTSEC=$$(cat /run/secrets/testpwd)
      echo $$TESTSEC
    secrets:
      - testpwd

secrets:
  testpwd:
    file: secret.txt

> docker stack deploy -c stack.yaml test
Creating network test_default
Creating secret test_testpwd
Creating service test_server
> docker service logs test_server
test_server.0.s68pzslhjbpw@docker-desktop    | Secret1234

Works just fine.