Appending content to Go templates within a certain section

I am interested in appending content to a Go template but within a certain section of the template. Since the template has a structure defined, whenever I try to append new content on executing the template, it appends the new content to the previously executed template content:

Example template:

 type Client struct {
	Opts    *ClientOpts
	Schemas *Schemas
	Types   map[string]Schema

	Container *{{.schema.Id}}Client
	
  }

Actual output:

type Client struct {
    	Opts    *ClientOpts
    	Schemas *Schemas
    	Types   map[string]Schema
    
    	Container *abcClient
    	
      }

type Client struct {
    	Opts    *ClientOpts
    	Schemas *Schemas
    	Types   map[string]Schema
    
    	Container *xyzClient
    	
      }
}

Desired output:

type Client struct {
    	Opts    *ClientOpts
    	Schemas *Schemas
    	Types   map[string]Schema
    
    	Container *abcClient
        Container *xyzClient
    	
      }

My current Go code looks like this:

func appendToFile(filename string, template *template.Template, schema client.Schema) error {
	output, err := os.OpenFile(filename, os.O_APPEND|os.O_WRONLY, 0600)
	if err != nil {
		panic(err)
	}
	defer output.Close()

	data := map[string]interface{}{
		"schema": schema,
	}
	err = template.Execute(output, data)
	return err

}

One solution I can think of is to make a seek every time to the previous appended content and write the new content there. But I am not sure how to do that in Go. Could someone provide me with a code snippet for that or suggest a better strategy?

Instead of appending the data, generate the output with one execution of the template:

package main

import (
    "fmt"
    "os"
    "text/template"
)

var t = template.Must(template.New("").Parse(` type Client struct {
    Opts    *ClientOpts
    Schemas *Schemas
    Types   map[string]Schema

{{range .}}
    Container *{{.schema.Id}}Client{{end}}

}
`))

type schema struct {
    Id string
}

func main() {
    data := []map[string]interface{}{
        {"schema": schema{Id: "abcClient"}},
        {"schema": schema{Id: "xyzClient"}},
    }
    if err := t.Execute(os.Stdout, data); err != nil {
        fmt.Println(err)
    }
}

Try it on the playground.