Q:

With Go's webserver where does the root of the website map onto the filesystem?

Where is the filesystem "root" of a Go net/http webserver. It doesn't seem to be in the directory the executable is in.
By "root" I mean the directory I would use for, say, the src attribute of an img, without any path. I don't plan to do this, but it would help me understand the structure if I knew.

A:

Introduction
In Go the net/http package is used to provide Web Server functionality. This is not a static fileserver, it is much more than that.
There is no filesystem "root" concept. The provided web server uses handlers to serve HTTP requests which are mapped to URLs. A handler is responsible to process an HTTP request and to setup and generate the response. A handler can be registered e.g. with the Handle() or HandleFunc() functions. The server can be started with the ListenAndServe() function.
Read the package documentation of net/http to understand the basic concepts and to get started. It also contains many small examples.
The Blog article Writing Web Applications is also helpful.
Static File Server
However, a static file server or "filesystem" functionality is provided, there is a FileServer() function in the http package which returns a Handler which serves static files. You can specify the "root" folder to serve static files from as a parameter of FileServer().
If you pass an absolute path to FileServer(), then there is no question what that means. If you provide a relative path, it is always interpreted in the context of the current or working directory. By default this is the folder you start the application from (the folder you're in when executing the go run ... command or the compiled executable binary).
Example:
http.Handle("/", http.FileServer(http.Dir("/tmp")))

This will setup a handler to serve files from the folder /tmp mapped to the root URL /. For example the response to the GET request "/mydoc.txt" will be the "/tmp/mydoc.txt" static file.
Complete application:
package main
import (
    "log"
    "net/http"
)
func main() {
    // Simple static webserver:
    http.Handle("/", http.FileServer(http.Dir("/tmp")))
    log.Fatal(http.ListenAndServe(":8080", nil))
}

You can do more complex mapping using the StripPrefix() function. Example:
// To serve a directory on disk (/tmp) under an alternate URL
// path (/tmpfiles/), use StripPrefix to modify the request
// URL's path before the FileServer sees it:
http.Handle("/tmpfiles/",
    http.StripPrefix("/tmpfiles/", http.FileServer(http.Dir("/tmp"))))

