docs: document cmd template function (#8929)

docs-v2/content/en/docs/environment/templating.md

@@ -43,4 +43,7 @@ List of variables that are available for templating:
   * `IMAGE_NAME2`, `IMAGE_REPO2`, `IMAGE_TAG2`, `IMAGE_DIGEST2` - the 2nd artifact's image name, tag, and sha256 digest
   * `IMAGE_NAME2`, `IMAGE_REPON`, `IMAGE_TAGN`, `IMAGE_DIGESTN` - the Nth artifact's image name, tag, and sha256 digest
 
-Default values can be specified using the `{{default "bar" .FOO}}` expression syntax, which results in "bar" if .FOO is nil or a zero value.
+### Template functions
+Besides the [built-in template functions](https://pkg.go.dev/text/template) in golang, skaffold also provides the following ones:
+- `default` : Default values can be specified using the `{{default "bar" .FOO}}` expression syntax, which results in "bar" if .FOO is nil or a zero value.
+- `cmd`: This allows users to use the result from external commands in template, for example `{{cmd "bash" "-c" "xxx xxx xxx"}}` can be used to execute bash script and get the result into the template.