fix(master): keep rollbacks of auto services until a new digest (#9)

Adds services.last_auto_digest so the poller starts one automatic update per
registry digest, and re-reads the service before starting it. Also records a
bare image's running version as tag latest, only records well-formed
digests, and shows a digest-less version unless a digest row of its tag was
first seen at or after it.

internal/e2e/versions_test.go

@@ -73,7 +73,12 @@ func TestPollStartsAutoJob(t *testing.T) {
 
 	p.Cycle(context.Background())
 
-	assert.Equal(t, slusk+":1.4.2", <-env.exec.started, "the agent gets repo:tag and resolves the digest itself")
+	select {
+	case ref := <-env.exec.started:
+		assert.Equal(t, slusk+":1.4.2", ref, "the agent gets repo:tag and resolves the digest itself")
+	case <-time.After(waitFor):
+		t.Fatal("the agent never received the job")
+	}
 	require.Equal(t, 1, env.store.jobCount())
 	job := env.job(t, 1)
 	assert.Equal(t, model.TriggerPoll, job.Trigger)
@@ -83,6 +88,7 @@ func TestPollStartsAutoJob(t *testing.T) {
 	assert.Equal(t, slusk+":1.4.2", svc.AvailableRef)
 	assert.Equal(t, "sha256:new", svc.AvailableDigest)
 	assert.Equal(t, model.SourcePoll, svc.AvailableSource)
+	assert.Equal(t, "sha256:new", svc.LastAutoDigest)
 
 	p.Cycle(context.Background())