Entrypoint validation fails if the entrypoint is a symlink

When the first item in the entrypoint array is a symbolic link (to /bin/busybox, for example), the entrypoint validation resolves this link relative to the build machine, rather than relative to the target rootfs. So, for example, the architecture validation would check /bin/busybox for compatibility, rather than /tmp/corteca-<id>/rootfs/bin/busybox, which would cause the validation to fail unless the build and target machines are the same architecture.

A full resolution is tricky, since we can't rely on standard lib functions (filepath.EvalSymlinks() exhibits the same behaviour). A partial fix is implemented below, but it currently only handles a single level of indirection:

func ValidateRootFS(rootfsPath string, targetArch string, appSettings configuration.AppSettings) error {
	entrypointPath := filepath.Join(rootfsPath, appSettings.Entrypoint[0])

	// TODO: handle more than a single symlink degree
	entrypointInfo, err := os.Lstat(entrypointPath)
	if err != nil {
		return err
	}
	if entrypointInfo.Mode()&os.ModeSymlink != 0 {
		// construct a rootfs-based path to the symlink target
		symPath, err := os.Readlink(entrypointPath)
		if err != nil {
			return err
		}
		entrypointPath = filepath.Join(rootfsPath, symPath)
	}
	if err := validateEntrypoint(entrypointPath, targetArch); err != nil {
		return err
	}

	if err := validateMounts(rootfsPath, appSettings.Runtime.Mounts); err != nil {
		return fmt.Errorf("runtime configuration error: %w", err)
	}

	return nil
}

This issue can be worked around by specifying the actual symlink targets in the entrypoint array. For example, if we wanted the container entry point to be a shell script, instead of specifying the entrypoint as ["/bin/sh", "/path/to/start.sh"], we could specify it as follows:

entrypoint:
  - /bin/busybox
  - sh
  - /path/to/start.sh