docker-compose, run a script after container has started?

I have a service that I am bringing up through Rancher via docker-compose. The issue I am running into is that I need to set a password after the container has been deployed.

The way rancher secrets work, is that I set my secret in and rancher will mount a volume on my container with a file containing my secret. I was hoping to be able to execute a script to grab that secret, and set it as a password on my config file.

I don't believe I have a way to get that secret in through the Dockerfile as I don't want the secret to be in git, so I'm left looking at doing it via docker-compose.

Does anyone know if this is possible?

The trick is to overwrite the compose COMMAND to perform whatever init action you need before calling the original command.

1. Add a script in your image that will perform the init work that you want like set password, change internal config files, etc. Let's call it init.sh. You add it to your image.

Dockerfile:

FROM: sourceimage:tag
COPY init.sh /usr/local/bin/
ENTRYPOINT []

The above overrides whatever ENTRYPOINT is defined in the sourceimage. That's to make this example simpler. Make sure you understand what the ENTRYPOINT is doing in the Dockerfile from the sourceimage and call it in the command: of the docker-compose.yml file.

docker-compose.yml:

services:
  myservice:
    image: something:tag
    ...
    command: sh -c "/usr/local/bin/init.sh && exec myexecutable"

It's important to use exec before calling the main command. That will install the command as the first process (PID1) which will make it receive signals like STOP, KILL (Ctrl-C on keyboard) or HUP.

This is the way I use for calling a script after a container is started without overriding the entrypoint.

In my example, I used it for initializing the replicaset of my local MongoDB

services:
  mongo:
    image: mongo:4.2.8
    hostname: mongo
    container_name: mongodb
    entrypoint: ["/usr/bin/mongod","--bind_ip_all","--replSet","rs0"]
    ports:
      - 27017:27017
  mongosetup:
    image: mongo:4.2.8
    depends_on:
      - mongo
    restart: "no"
    entrypoint: [ "bash", "-c", "sleep 10 && mongo --host mongo:27017 --eval 'rs.initiate()'"]      

• In the first part, I simply launch my service (mongo)
• The second service use a "bash" entry point AND a restart: no <= important

I also use a depends_on between service and setup service for manage the launch order.