fix: Use busybox:musl instead of busybox:latest

Turns out, busybox:latest is not (always) statically linked. In particular, arm64 images.

README.md

@@ -67,7 +67,7 @@ a debugging "sidecar" container that _feels_ like a `docker exec` session to the
 - No extra volumes or copying of debugging tools is needed.
 - The debugging tools **_are_** available in the target container.
 
-By default, the `busybox:latest` image is used for the debugger sidecar, but you can override it
+By default, the `busybox:musl` (statically compiled) image is used for the debugger sidecar, but you can override it
 with the `--image` flag. Combining this with the superpower of Nix and [Nixery](https://nixery.dev/),
 you can get all your favorite tools by simply listing them in the image name:
 

cmd/exec/exec.go

@@ -15,7 +15,7 @@ import (
 )
 
 const (
-	defaultToolkitImage = "docker.io/library/busybox:latest"
+	defaultToolkitImage = "docker.io/library/busybox:musl"
 
 	schemaContainerd = "containerd://"
 	schemaDocker     = "docker://"
@@ -120,7 +120,7 @@ func NewCommand(cli cliutil.CLI) *cobra.Command {
 		&opts.image,
 		"image",
 		defaultToolkitImage,
-		`Debugging toolkit image (hint: use "busybox" or "nixery.dev/shell/vim/ps/tool3/tool4/...")`,
+		`Debugging toolkit image (hint: use "busybox:musl" or "nixery.dev/shell/vim/ps/tool3/tool4/...")`,
 	)
 	flags.BoolVarP(
 		&opts.stdin,