Fix ADD remote files issue. Hitting the cache for different file's content (#3847)

Force reading of downloaded file contents to obtain the checksum used by cache validation.

Docker-DCO-1.1-Signed-off-by: German Del Zotto <germ@ndz.com.ar> (github: GermanDZ)
Rewrite with a simpler approach (just sum the downloaded file during the copy)

Docker-DCO-1.1-Signed-off-by: Abel Muiño <amuino@gmail.com> (github: amuino)
Remove fmt.Println

Docker-DCO-1.1-Signed-off-by: Abel Muiño <amuino@gmail.com> (github: amuino)
Remove unneeded second download.

Docker-DCO-1.1-Signed-off-by: Abel Muiño <amuino@gmail.com> (github: amuino)