Update hugo render-image script

[Summon528]

Use path.Join so that it will render correctly on Windows (path.Join will convert Windows backslash to forward slash)

[dreamworld]

Encountered the same problem on Windows due to .File.Dir output

https://gohugo.io/variables/files/

Note that the path separator (\ or /) could be dependent on the operating system.

[zoni]

Thank you for this!