templates: don't fail table headers that use the slice function

cli/command/formatter/container_test.go

@@ -387,6 +387,10 @@ size: 0B
 			context:  Context{Format: NewContainerFormat(`table {{split .Image ":"}}`, false, false)},
 			expected: "IMAGE\n[ubuntu]\n[ubuntu]\n",
 		},
+		{
+			context:  Context{Format: NewContainerFormat(`table {{slice .Names 0 6}}`, false, false)},
+			expected: "NAMES\nfoobar\nfoobar\n",
+		},
 	}
 
 	containers := []container.Summary{

templates/templates.go

@@ -58,6 +58,12 @@ var HeaderFunctions = template.FuncMap{
 	"truncate": func(v string, _ int) string {
 		return v
 	},
+	"slice": func(v string, _ ...int) string {
+		// slicing the column name would fail if the indexes are out of
+		// range for the header (e.g. "{{slice .ID 0 15}}"), so return
+		// the column name as-is.
+		return v
+	},
 }
 
 // Parse creates a new anonymous template with the basic functions

templates/templates_test.go

@@ -124,6 +124,14 @@ func TestHeaderFunctions(t *testing.T) {
 			doc:      "truncate",
 			template: `{{ truncate . 2}}`,
 		},
+		{
+			doc:      "slice",
+			template: `{{ slice . 2}}`,
+		},
+		{
+			doc:      "slice out of range",
+			template: `{{ slice . 0 15}}`,
+		},
 	}
 
 	for _, tc := range tests {