Session report collection and report templates (#981)

* wip: notification stats

* make report notifications optional

* linting/documentation fixes

* linting/documentation fixes

* merge types.Container and container.Interface

* smaller naming/format fixes

* use typed image/container IDs

* simplify notifier and update tests

* add missed doc comments

* lint fixes

* remove unused constructors

* rename old/new current/latest

pkg/session/container_status.go

@@ -0,0 +1,82 @@
+package session
+
+import wt "github.com/containrrr/watchtower/pkg/types"
+
+// State indicates what the current state is of the container
+type State int
+
+// State enum values
+const (
+	// UnknownState is only used to represent an uninitialized State value
+	UnknownState State = iota
+	SkippedState
+	ScannedState
+	UpdatedState
+	FailedState
+	FreshState
+	StaleState
+)
+
+// ContainerStatus contains the container state during a session
+type ContainerStatus struct {
+	containerID   wt.ContainerID
+	oldImage      wt.ImageID
+	newImage      wt.ImageID
+	containerName string
+	imageName     string
+	error
+	state State
+}
+
+// ID returns the container ID
+func (u *ContainerStatus) ID() wt.ContainerID {
+	return u.containerID
+}
+
+// Name returns the container name
+func (u *ContainerStatus) Name() string {
+	return u.containerName
+}
+
+// CurrentImageID returns the image ID that the container used when the session started
+func (u *ContainerStatus) CurrentImageID() wt.ImageID {
+	return u.oldImage
+}
+
+// LatestImageID returns the newest image ID found during the session
+func (u *ContainerStatus) LatestImageID() wt.ImageID {
+	return u.newImage
+}
+
+// ImageName returns the name:tag that the container uses
+func (u *ContainerStatus) ImageName() string {
+	return u.imageName
+}
+
+// Error returns the error (if any) that was encountered for the container during a session
+func (u *ContainerStatus) Error() string {
+	if u.error == nil {
+		return ""
+	}
+	return u.error.Error()
+}
+
+// State returns the current State that the container is in
+func (u *ContainerStatus) State() string {
+	switch u.state {
+	case SkippedState:
+		return "Skipped"
+	case ScannedState:
+		return "Scanned"
+	case UpdatedState:
+		return "Updated"
+	case FailedState:
+		return "Failed"
+	case FreshState:
+		return "Fresh"
+	case StaleState:
+		return "Stale"
+	default:
+		return "Unknown"
+	}
+}