Allow/document prioritizing offline images in services

[tonistiigi]

#23790 added support for this as an environment variable DOCKER_SERVICE_PREFER_OFFLINE_IMAGE to support our integration tests.

We should figure out if we want to:

• document this
• make it configurable in some other way
• use digest or image IDs to avoid pulling if local image is present.

I'd prefer not to rush this for 1.12 timeframe.

@vdemeester

[vdemeester]

I'd prefer not to rush this for 1.12 timeframe.

Sounds more than good to me 👼

[thaJeztah]

use digest or image IDs to avoid pulling if local image is present.

Resolving digests, and pinning services to that digest was implemented in #28173

Now, if an image with the resolved digest is already present locally, no pull is performed;

DEBU[43610] form data: {"EndpointSpec":{},"Mode":{"Replicated":{}},"Name":"web","TaskTemplate":{"ContainerSpec":{"DNSConfig":{},"Image":"nginx:alpine"},"ForceUpdate":0,"Placement":{},"Resources":{"Limits":{},"Reservations":{}},"RestartPolicy":{}},"UpdateConfig":{"FailureAction":"pause","MaxFailureRatio":0,"Parallelism":1}}
DEBU[43610] Increasing token expiration to: 60 seconds
DEBU[43612] pinning image nginx:alpine by digest: nginx:alpine@sha256:5c6be3514a00db611371806c5aeb510dfd285a4b8261c6e5c3323387e396b66b
DEBU[43612] Service r5b6fv0f8t1krul3dg48hw4hd was scaled up from 0 to 1 instances

@tonistiigi should we close this issue, or do you want to document DOCKER_SERVICE_PREFER_OFFLINE_IMAGE ?

[tonistiigi]

@thaJeztah This itself doesn't help for tests. It could only work if we add a digest based busybox to frozen images(can't be done with docker load) and then modify every test to use that reference(seems very easy to break).

[justincormack]

I had no idea this existed. Using an environment variable is a terrible way to set this. If it is going to exist can we have it in a config file please. But it seems something that should not be in the global (especially per daemon) config, but a property of the service.