how do you manage secret values with docker-compose v3.1?

You can read the corresponding section from the official documentation.

To use secrets you need to add two things into your docker-compose.yml file. First, a top-level secrets: block that defines all of the secrets. Then, another secrets: block under each service that specifies which secrets the service should receive.

As an example, create the two types of secrets that Docker will understand: external secrets and file secrets.

1. Create an ‘external’ secret using docker secret create

First thing: to use secrets with Docker, the node you are on must be part of a swarm.

$ docker swarm init

Next, create an ‘external’ secret:

$ echo "This is an external secret" | docker secret create my_external_secret -

(Make sure to include the final dash, -. It’s easy to miss.)

2. Write another secret into a file

$ echo "This is a file secret." > my_file_secret.txt

3. Create a docker-compose.yml file that uses both secrets

Now that both types of secrets are created, here is the docker-compose.yml file that will read both of those and write them to the web service:

version: '3.1'

services:
  web:
    image: nginxdemos/hello
    secrets:                    # secrets block only for 'web' service
     - my_external_secret
     - my_file_secret

secrets:                        # top level secrets block
  my_external_secret:
    external: true
  my_file_secret:
    file: my_file_secret.txt

Docker can read secrets either from its own database (e.g. secrets made with docker secret create) or from a file. The above shows both examples.

4. Deploy your test stack

Deploy the stack using:

$ docker stack deploy --compose-file=docker-compose.yml secret_test

This will create one instance of the web service, named secret_test_web.

5. Verify that the container created by the service has both secrets

Use docker exec -ti [container] /bin/sh to verify that the secrets exist.

(Note: in the below docker exec command, the m2jgac... portion will be different on your machine. Run docker ps to find your container name.)

$ docker exec -ti secret_test_web.1.m2jgacogzsiaqhgq1z0yrwekd /bin/sh

# Now inside secret_test_web; secrets are contained in /run/secrets/
root@secret_test_web:~$ cd /run/secrets/

root@secret_test_web:/run/secrets$ ls
my_external_secret  my_file_secret

root@secret_test_web:/run/secrets$ cat my_external_secret
This is an external secret

root@secret_test_web:/run/secrets$ cat my_file_secret
This is a file secret.

If all is well, the two secrets we created in steps 1 and 2 should be inside the web container that was created when we deployed our stack.