Using docker watch feature for CI/CD

I'm interested in the "experimental" docker compose watch feature: [1] [2].

The two actions available are "sync" and "rebuild" (which is actually rebuild + restart).

I need this for continuous deployment: all I want is docker to detect that I've changed the .env or docker-compose.yml file (by updating the image version to be pulled) - and to restart the container. No rebuilding is necessary, as I'm not building the image in the first place but rather pulling it from a docker registry.

How can I do that?

I've tried many things, and it does not seem possible to update/restart the containers when modifying either .env or docker-compose.yml. For some reason, changing these files restarts the container, but does not honor the changes.

However, you can achieve what you want, but instead of modifying .env or docker-compose.yml, it requires defining an intermediate Dockerfile and make changes to that file directly. To me, it's even easier because all changes are localized to that Dockerfile. And you can define the following in your docker-compose.yml:

services:
  myapp:
    build:
      context: ./myapp
    x-develop:
      watch:
        - action: rebuild
          path: ./myapp/Dockerfile

Anyway, here's an example below with envoyproxy/envoy image from DockerHub.

Example

With the following directory structure:

$ tree
.
├── docker-compose.yml
└── envoy
    ├── Dockerfile
    └── envoy.yaml # it does not matter, it's just a dummy config file

And the following files:

• envoy/Dockerfile

FROM    envoyproxy/envoy:v1.27.0

ENV     ENVOY_UID=1234

• docker-compose.yml:

version: '3'

services:
  envoy:
    build:
      context: ./envoy
    ports:
      - "9901:9901"
    volumes:
      - ./envoy/envoy.yaml:/etc/envoy/envoy.yaml
    x-develop:
      watch:
        - action: rebuild
          path: ./envoy/Dockerfile

Any changes to envoy/Dockerfile you've asked in your question, like:

• updating the image (FROM directive): equivalent of changing the image in docker-compose.yml in your question
• updating/adding/removing an environment variable (ENV directive): equivalent of changing the .env in your question

will trigger a rebuild of the image, and restart with the new values.

You can test it works by doing:

$ docker compose up -d 2>/dev/null

$ docker compose alpha watch

And in another window:

$ # check envoy version (matches the version in Dockerfile)
$ curl -s http://localhost:9901/server_info | jq -r .version | cut -d / -f 2
1.27.0

$ # check ENVOY_UID env var
$ docker inspect 76803907-using-docker-watch-feature-for-ci-cd-envoy | jq -r '.[0].Config.Env' | grep 'ENVOY_UID'
  "ENVOY_UID=1234"

$ # make some changes (image version, env var)
$ sed -i -e 's/v1.27.0/v1.21.0/' -e 's/ENVOY_UID=1234/ENVOY_UID=5678/' envoy/Dockerfile

$ # wait a few seconds while docker compose is restarting the container...
$ sleep 5

$ # check envoy version has been updated!
$ curl -s http://localhost:9901/server_info | jq -r .version | cut -d / -f 2
1.21.0

$ # check ENVOY_UID env var has been updated!
$ docker inspect 76803907-using-docker-watch-feature-for-ci-cd-envoy | jq -r '.[0].Config.Env' | grep 'ENVOY_UID'
  "ENVOY_UID=5678"