Golang正则表达式匹配并替换某个字符串后的第一个匹配项

Following up on my previous question about using Golang's regex to replace between strings. I now have a bit of complexity added to it. Here is what the contexts of my file looks like:

foo:
    blahblah
    MYSTRING=*
bar:
    blah
    blah
    MYSTRING=*

MYSTRING=

var re = regexp.MustCompile(`(MYSTRING=).*`)
s := re.ReplaceAllString(content, `${1}stringofmychoice`)

But now I need to match and replace only after a certain occurrence. So that the contents of my file can look something like this:

foo:
    blahblah
    MYSTRING=foostring
bar:
    blah
    blah
    MYSTRING=barstring

ReplaceAllString

docker-compose.ymlMYSTRINGfoobar