mkctr: add support for volumes

[raggi]

Volumes are the conventional way to specify state directories.

[bradfitz]

This part shouldn't be conditional on len(args) > 0 (setting an entrypoint)

I noted this in my upcoming PR to add env var support too.

[bradfitz]

I'll handle this fix in my upcoming PR.

[bradfitz]

this requires Go 1.24. But the go.mod still says 1.23.

[bradfitz]

Fixed go.mod in #31

[Copilot]

Pull request overview

This PR adds support for specifying OCI image volumes through a new --volumes command-line flag. Volumes provide a conventional way to declare state directories in container images.

Key Changes:

• Added volumes field to the buildParams struct to store volume definitions
• Implemented command-line flag parsing to accept comma-separated volume paths
• Integrated volumes into the OCI image configuration during the build process

Comments suppressed due to low confidence (1)

mkctr.go:325

• Volumes are only being set when command args are present (inside the if args := flag.Args(); len(args) > 0 block). This means that if no command arguments are provided, the volumes will not be applied to the image even if the --volumes flag was specified.

Consider moving the volumes configuration outside of the args conditional, or include volumes in the Config regardless of whether args are present. Volumes are typically independent of the command specification.

		if args := flag.Args(); len(args) > 0 {
			img, err = mutate.Config(img, v1.Config{
				Cmd:     args,
				Volumes: bp.volumes,
			})
			if err != nil {
				return err
			}
		}

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[Copilot]

The volume parsing logic doesn't handle empty strings correctly. When the volumes flag is empty (default value ""), strings.SplitSeq will still yield one iteration with an empty string, which will be added to the map after TrimSpace. This results in an empty volume key being added to the volumes map.

Consider adding a check to skip empty strings after trimming, or check if the volumes string is empty before attempting to parse it.

Suggested change

	if vols == nil {
	vols = make(map[string]struct{})
	}
	vols[strings.TrimSpace(vol)] = struct{}{}
	vol = strings.TrimSpace(vol)
	if vol == "" {
	continue
	}
	if vols == nil {
	vols = make(map[string]struct{})
	}
	vols[vol] = struct{}{}