fix: recreate containers only if spec changed by comparing spec hash

internal/cli/client/container.go

@@ -40,17 +40,22 @@ func (cli *Client) CreateContainer(
 	}
 	containerName := fmt.Sprintf("%s-%s", spec.Name, suffix)
 
-	// TODO: calculate the spec hash and set it as a label to detect changes in the service spec.
+	specHash, err := spec.ImmutableHash()
+	if err != nil {
+		return resp, fmt.Errorf("calculate immutable hash for service spec: %w", err)
+	}
+
 	config := &container.Config{
 		Cmd:        spec.Container.Command,
 		Entrypoint: spec.Container.Entrypoint,
 		Hostname:   containerName,
 		Image:      spec.Container.Image,
 		Labels: map[string]string{
-			api.LabelServiceID:   serviceID,
-			api.LabelServiceName: spec.Name,
-			api.LabelServiceMode: spec.Mode,
-			api.LabelManaged:     "",
+			api.LabelServiceID:       serviceID,
+			api.LabelServiceName:     spec.Name,
+			api.LabelServiceMode:     spec.Mode,
+			api.LabelServiceSpecHash: specHash,
+			api.LabelManaged:         "",
 		},
 	}
 	if spec.Mode == "" {
@@ -335,3 +340,26 @@ func (cli *Client) RemoveContainer(
 
 	return nil
 }
+
+type ContainerSpecStatus string
+
+const ContainerUpToDate ContainerSpecStatus = "up-to-date"
+const ContainerNeedsUpdate ContainerSpecStatus = "needs-update"
+const ContainerNeedsRecreate ContainerSpecStatus = "needs-recreate"
+
+func CompareContainerToSpec(ctr api.Container, spec api.ServiceSpec) (ContainerSpecStatus, error) {
+	specHash, err := spec.ImmutableHash()
+	if err != nil {
+		return "", fmt.Errorf("calculate immutable hash for service spec: %w", err)
+	}
+
+	// Is the hash label is unset, there is no easy way to compare its configuration with the spec,
+	// so let's recreate as well.
+	if ctr.Config.Labels[api.LabelServiceSpecHash] != specHash {
+		return ContainerNeedsRecreate, nil
+	}
+
+	// TODO: compare mutable properties such as memory or CPU limits when they are implemented.
+
+	return ContainerUpToDate, nil
+}