feat(docs): add template preview (#1777)

pkg/notifications/preview/tplprev.go

@@ -0,0 +1,36 @@
+package preview
+
+import (
+	"fmt"
+	"strings"
+	"text/template"
+
+	"github.com/containrrr/watchtower/pkg/notifications/preview/data"
+	"github.com/containrrr/watchtower/pkg/notifications/templates"
+)
+
+func Render(input string, states []data.State, loglevels []data.LogLevel) (string, error) {
+
+	data := data.New()
+
+	tpl, err := template.New("").Funcs(templates.Funcs).Parse(input)
+	if err != nil {
+		return "", fmt.Errorf("failed to parse template: %e", err)
+	}
+
+	for _, state := range states {
+		data.AddFromState(state)
+	}
+
+	for _, level := range loglevels {
+		data.AddLogEntry(level)
+	}
+
+	var buf strings.Builder
+	err = tpl.Execute(&buf, data)
+	if err != nil {
+		return "", fmt.Errorf("failed to execute template: %e", err)
+	}
+
+	return buf.String(), nil
+}