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/agent_master_test.go

@@ -91,6 +91,18 @@ func (m *memStore) SetPollError(_ context.Context, ids []int64, msg string) erro
 	return nil
 }
 
+func (m *memStore) SetLastAutoDigest(_ context.Context, id int64, digest string) error {
+	m.mu.Lock()
+	defer m.mu.Unlock()
+	svc, ok := m.svcRows[id]
+	if !ok {
+		return model.ErrNotFound
+	}
+	svc.LastAutoDigest = digest
+	m.svcRows[id] = svc
+	return nil
+}
+
 // seedService adds a service row that jobs can be triggered for.
 func (m *memStore) seedService(svc model.Service) {
 	m.mu.Lock()