`podman_build` pushes a new tag on every build, even when the image is unchanged

[azmeuk]

podman_build and podman_build_with_restart both push $EXPECTED_REF. In normal mode Tilt sets that to tilt-build-<unix-timestamp>, so each build pushes a tag that nothing will ever reuse, even when podman rebuilt nothing and the image is byte-for-byte the same.

The registry garbage collector can't drop anything, because every old layer is still reachable from a tag. It only grows from there.

The two entry points don't get off equally lightly. podman_build_with_restart just leaves the tags behind. Plain podman_build deploys the timestamped ref as well, so the pods get rolled on every build.

Steps to reproduce

kind, with the local registry from https://kind.sigs.k8s.io/docs/user/local-registry/. Then three files.

Containerfile:

FROM docker.io/alpine:3.21
CMD ["sleep", "infinity"]

Tiltfile:

load('ext://podman', 'podman_build')

podman_build('localhost:5000/mre', '.', deps=['trigger'])
k8s_yaml('mre.yaml')

mre.yaml:

apiVersion: v1
kind: Namespace
metadata:
  name: podman-tag-mre
---
apiVersion: apps/v1
kind: Deployment
metadata:
  name: mre
  namespace: podman-tag-mre
spec:
  replicas: 1
  selector:
    matchLabels: {app: mre}
  template:
    metadata:
      labels: {app: mre}
    spec:
      containers:
        - name: mre
          image: localhost:5000/mre

Run tilt up, then touch trigger three times a few seconds apart. That's four builds counting the initial one, and here is what the registry and the cluster hold afterwards:

$ curl -s http://localhost:5000/v2/mre/tags/list
{"name":"mre","tags":["tilt-build-1788467962","tilt-build-1788468004","tilt-build-1788468012","tilt-build-1788468020"]}

# Docker-Content-Digest of each of those tags
# tilt-build-1788467962  sha256:6aa8e6fbcccfa4fbcc202727409061c465322d443480d724044a878a22ce683c
# tilt-build-1788468004  sha256:6aa8e6fbcccfa4fbcc202727409061c465322d443480d724044a878a22ce683c
# tilt-build-1788468012  sha256:6aa8e6fbcccfa4fbcc202727409061c465322d443480d724044a878a22ce683c
# tilt-build-1788468020  sha256:6aa8e6fbcccfa4fbcc202727409061c465322d443480d724044a878a22ce683c

$ kubectl get rs -n podman-tag-mre -o custom-columns=RS:.metadata.name,REPLICAS:.spec.replicas,IMAGE:.spec.template.spec.containers[0].image
RS               REPLICAS   IMAGE
mre-59f88c7f57   1          localhost:5000/mre:tilt-build-1788468020
mre-6bf486c8c    0          localhost:5000/mre:tilt-build-1788468012
mre-8fbfb6566    0          localhost:5000/mre:tilt-build-1788467962
mre-dc6b66b9c    0          localhost:5000/mre:tilt-build-1788468004

Context

• Tilt v0.37.6
• podman 5.8.6, rootless, no local Docker involved
• tilt-extensions at a8701c0
• kind 1.35.5 with a local registry