Add sprig template functions to label templates

Add the sprig template function library, giving users access to
100+ utility functions (string manipulation, math, date, etc.)
in their Docker label templates.

This enables workarounds like trimPrefix for the leading slash in
container names from the Docker API, e.g.:
  {{trimPrefix "/" (index .Names 0)}}

Closes #648

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: lucaslorentz

generator/containers.go

@@ -1,8 +1,6 @@
 package generator
 
 import (
-	"strings"
-
 	"github.com/docker/docker/api/types"
 	"github.com/lucaslorentz/caddy-docker-proxy/v2/caddyfile"
 	"go.uber.org/zap"
@@ -11,21 +9,11 @@ import (
 func (g *CaddyfileGenerator) getContainerCaddyfile(container *types.Container, logger *zap.Logger) (*caddyfile.Container, error) {
 	caddyLabels := g.filterLabels(container.Labels)
 
-	trimContainerNames(container)
-
 	return labelsToCaddyfile(caddyLabels, container, func() ([]string, error) {
 		return g.getContainerIPAddresses(container, logger, true)
 	})
 }
 
-// trimContainerNames removes the leading slash from container names.
-// The Docker API returns names prefixed with "/" (e.g. "/mycontainer").
-func trimContainerNames(container *types.Container) {
-	for i, name := range container.Names {
-		container.Names[i] = strings.TrimPrefix(name, "/")
-	}
-}
-
 func (g *CaddyfileGenerator) getContainerIPAddresses(container *types.Container, logger *zap.Logger, onlyIngressIps bool) ([]string, error) {
 	ips := []string{}
 

generator/containers_test.go

@@ -24,7 +24,7 @@ func TestContainers_TemplateData(t *testing.T) {
 				},
 			},
 			Labels: map[string]string{
-				fmtLabel("%s"):               "{{index .Names 0}}.testdomain.com",
+				fmtLabel("%s"):               "{{trimPrefix \"/\" (index .Names 0)}}.testdomain.com",
 				fmtLabel("%s.reverse_proxy"): "{{(index .NetworkSettings.Networks \"caddy-network\").IPAddress}}:5000/api",
 			},
 		},

generator/labels.go

@@ -1,45 +1,38 @@
 package generator
 
 import (
+	"sort"
 	"strconv"
 	"strings"
-	"text/template"
-	"sort"
 
+	"github.com/Masterminds/sprig/v3"
 	"github.com/lucaslorentz/caddy-docker-proxy/v2/caddyfile"
 )
 
 type targetsProvider func() ([]string, error)
 
 func labelsToCaddyfile(labels map[string]string, templateData interface{}, getTargets targetsProvider) (*caddyfile.Container, error) {
-	funcMap := template.FuncMap{
-		"upstreams": func(options ...interface{}) (string, error) {
-			targets, err := getTargets()
-			sort.Strings(targets)
-			transformed := []string{}
-			for _, target := range targets {
-				for _, param := range options {
-					if protocol, isProtocol := param.(string); isProtocol {
-						target = protocol + "://" + target
-					} else if port, isPort := param.(int); isPort {
-						target = target + ":" + strconv.Itoa(port)
-					}
+	funcMap := sprig.TxtFuncMap()
+	funcMap["upstreams"] = func(options ...interface{}) (string, error) {
+		targets, err := getTargets()
+		sort.Strings(targets)
+		transformed := []string{}
+		for _, target := range targets {
+			for _, param := range options {
+				if protocol, isProtocol := param.(string); isProtocol {
+					target = protocol + "://" + target
+				} else if port, isPort := param.(int); isPort {
+					target = target + ":" + strconv.Itoa(port)
 				}
-				transformed = append(transformed, target)
 			}
-			sort.Strings(transformed)
-			return strings.Join(transformed, " "), err
-		},
-		"http": func() string {
-			return "http"
-		},
-		"https": func() string {
-			return "https"
-		},
-		"h2c": func() string {
-			return "h2c"
-		},
+			transformed = append(transformed, target)
+		}
+		sort.Strings(transformed)
+		return strings.Join(transformed, " "), err
 	}
+	funcMap["http"] = func() string { return "http" }
+	funcMap["https"] = func() string { return "https" }
+	funcMap["h2c"] = func() string { return "h2c" }
 
 	return caddyfile.FromLabels(labels, templateData, funcMap)
 }