cmd/create: Require -y to pull an image when not connected to a terminal

It doesn't make sense to show the image download prompt when the
standard input and output streams are redirected to something other than
a terminal device.

During such non-interactive use, there's no way for the user to see the
prompt and the size of the image and then make a decision based on them.
The decision has to be made differently and earlier.  The user will
either never download or always download or will use 'skopeo inspect'
to decide for themself.

Secondly, when the input and output are not connected to a terminal, the
terminal escape sequences and the terminal-specific ioctl(2) requests
used to show the prompt won't work anyway.

#1428

src/cmd/create.go

@@ -730,6 +730,16 @@ func pullImage(image, release, authFile string) (bool, error) {
 	}
 
 	if promptForDownload {
+		if !term.IsTerminal(os.Stdin) || !term.IsTerminal(os.Stdout) {
+			var builder strings.Builder
+			fmt.Fprintf(&builder, "image required to create toolbox container.\n")
+			fmt.Fprintf(&builder, "Use option '--assumeyes' to download the image.\n")
+			fmt.Fprintf(&builder, "Run '%s --help' for usage.", executableBase)
+
+			errMsg := builder.String()
+			return false, errors.New(errMsg)
+		}
+
 		shouldPullImage = showPromptForDownload(imageFull)
 	}