Golang templates : how to define array in a variable?

You can use the sprig library to help here (I'm using this for helm templates - which are go templates, but helm has sprig builtin)

$myList := list  0 1 2
$new = append $myList 3

From Sprig Lists

there is no built-in way to do what you want to achieve. See the arguments on what you can do with the arguments and the pipeline.

But you could easily define your own function to achieve your goal:

package main

import (
    "html/template"
    "os"
)

func main() {
tmpl := `
{{ $slice := mkSlice "a" 5 "b" }}
{{ range $slice }}
     {{ . }}
{{ end }}
`
    funcMap := map[string]interface{}{"mkSlice": mkSlice}
    t := template.New("demo").Funcs(template.FuncMap(funcMap))
    template.Must(t.Parse(tmpl))
    t.ExecuteTemplate(os.Stdout, "demo", nil)
}

func mkSlice(args ...interface{}) []interface{} {
    return args
}

Playground.