Parse content of files in subdirectories with golang templates

Question

We have structure of files like

dir1/file.json
fir2/file.json
dir3/file.json
test.tmpl

How to include content of all files like

{{range . }}
   {{ include . }}
 {{end}}

I tried to use examples from https://www.krakend.io/docs/configuration/templates/ But it doesn't work like i expected.