Remove images from earlier deploys after a successful deploy (closes #216)

Every deploy built and tagged a new image and nothing removed the old ones, so disk use grew with each push. After a successful deploy, upaas now removes the app's old `upaas-<app>:<N>` tags by name, without force, keeping the current image and the previous one that rollback starts. Docker deletes an image only when no other tag or container still uses it, so an image shared with another app stays. A failed removal is a warning in the deployment log, not a failed deploy. The post-deploy step is one function, tested against a fake Docker API.

Judgement call: untagged images from other stages of a multi-stage build stay; they are the build cache.
On the first deploy after upgrading, all older images of that app are removed at once.

Model: opus-5-5

internal/docker/client.go

@@ -537,6 +537,55 @@ func (c *Client) RemoveImage(ctx context.Context, imageID ImageID) error {
 	return nil
 }
 
+// ListImageTags returns the tags in the given repository, such as
+// "upaas-myapp:12" in "upaas-myapp", each with the ID of its image.
+// Tags the same image has in other repositories are left out.
+func (c *Client) ListImageTags(
+	ctx context.Context,
+	repository string,
+) (map[string]ImageID, error) {
+	if c.docker == nil {
+		return nil, ErrNotConnected
+	}
+
+	images, err := c.docker.ImageList(ctx, image.ListOptions{
+		Filters: filters.NewArgs(filters.Arg("reference", repository)),
+	})
+	if err != nil {
+		return nil, fmt.Errorf("failed to list images: %w", err)
+	}
+
+	tags := make(map[string]ImageID)
+
+	for _, img := range images {
+		for _, tag := range img.RepoTags {
+			if strings.HasPrefix(tag, repository+":") {
+				tags[tag] = ImageID(img.ID)
+			}
+		}
+	}
+
+	return tags, nil
+}
+
+// RemoveImageTag removes a tag such as "upaas-myapp:12", without force.
+// Docker then deletes the image, and the untagged images it was built on,
+// only if no other tag and no container still uses it.
+func (c *Client) RemoveImageTag(ctx context.Context, tag string) error {
+	if c.docker == nil {
+		return ErrNotConnected
+	}
+
+	_, err := c.docker.ImageRemove(ctx, tag, image.RemoveOptions{
+		PruneChildren: true,
+	})
+	if err != nil && !client.IsErrNotFound(err) {
+		return fmt.Errorf("failed to remove image tag %s: %w", tag, err)
+	}
+
+	return nil
+}
+
 func (c *Client) performBuild(
 	ctx context.Context,
 	opts BuildImageOptions,