Add option to wait for docker before starting

[c-w]

What does this pull request do?

This pull request adds a new configuration option DOZZLE_WAIT_FOR_DOCKER which if set will defer the server start until the connection to docker is healthy.

Why is this useful?

In some environments, the docker connection isn't via a socket file but instead via tcp, for example if we're connecting to a remote docker host or if we're using a proxy to only allow selective access to docker APIs for security reasons. In these scenarios, the docker host may or may not be ready to serve requests at the time that dozzle starts. To simplify deployments in such an environment, it's very useful if dozzle can "self heal" and only start talking to the docker host once the docker host is ready for connections.

Of course it would also be possible to perform an external health check before starting the dozzle container, but this adds non-trivial complexity to the orchestration especially as the the dozzle container doesn't contain a shell so we couldn't for example override the entrypoint to a loop like sh -c "while ! exec /dozzle; do sleep 5; done". Other tools that talk to the docker host (e.g. Traefik) implement this sort of "self heal" mechanism so I believe it would be neat if dozzle could also do this, especially as the added code complexity is quite minor.

How can I test these changes?

Create a docker-compose file that defines dozzle and the docker socket proxy:

version: "2.3"

services:

  dozzle:
    build: .
    environment:
      DOZZLE_NO_ANALYTICS: "1"
      DOZZLE_WAIT_FOR_DOCKER_SECONDS: "120"
      DOCKER_HOST: "tcp://docker-proxy:2375"
    ports:
      - "8080:8080"

  docker-proxy:
    image: tecnativa/docker-socket-proxy
    environment:
      CONTAINERS: "1"
    volumes:
      - /var/run/docker.sock:/var/run/docker.sock:ro

Now start dozzle via docker-compose up --build dozzle and you will notice that the server waits to start up since the docker connection can't be established. Now start the docker socket proxy via docker-compose up -d docker-proxy and you will notice that dozzle now starts the server.

[c-w]

Note that I had to push a change to the dev.yml Github Action as it was failing for me since pull requests from forks don't have access to the secrets. Instead of failing the action, we now skip it for any pull request from a fork.

[amir20]

Interesting use case. I never had it happen. But it makes sense. I'll review later.