docker: set force=true on remove image to handle images referenced by multiple tags

.changelog/15962.txt

@@ -0,0 +1,3 @@
+```release-note:bug
+docker: Fixed a bug where images referenced by multiple tags would not be GC'd
+```

drivers/docker/coordinator.go

@@ -59,7 +59,7 @@ func (p *pullFuture) set(imageID string, err error) {
 type DockerImageClient interface {
 	PullImage(opts docker.PullImageOptions, auth docker.AuthConfiguration) error
 	InspectImage(id string) (*docker.Image, error)
-	RemoveImage(id string) error
+	RemoveImageExtended(id string, opts docker.RemoveImageOptions) error
 }
 
 // LogEventFn is a callback which allows Drivers to emit task events.
@@ -320,7 +320,9 @@ func (d *dockerCoordinator) removeImageImpl(id string, ctx context.Context) {
 	d.imageLock.Unlock()
 
 	for i := 0; i < 3; i++ {
-		err := d.client.RemoveImage(id)
+		err := d.client.RemoveImageExtended(id, docker.RemoveImageOptions{
+			Force: true, // necessary to GC images referenced by multiple tags
+		})
 		if err == nil {
 			break
 		}

drivers/docker/coordinator_test.go

@@ -48,7 +48,7 @@ func (m *mockImageClient) InspectImage(id string) (*docker.Image, error) {
 	}, nil
 }
 
-func (m *mockImageClient) RemoveImage(id string) error {
+func (m *mockImageClient) RemoveImageExtended(id string, options docker.RemoveImageOptions) error {
 	m.lock.Lock()
 	defer m.lock.Unlock()
 	m.removed[id]++

website/content/docs/drivers/docker.mdx

@@ -959,7 +959,8 @@ host system.
     here](https://golang.org/pkg/time/#ParseDuration), that defaults to `3m`.
     The delay controls how long Nomad will wait between an image being unused
     and deleting it. If a task is received that uses the same image within
-    the delay, the image will be reused.
+    the delay, the image will be reused. If an image is referenced by more than
+    one tag, `image_delay` may not work correctly.
 
   - `container` - Defaults to `true`. This option can be used to disable Nomad
     from removing a container when the task exits. Under a name conflict,