tmpl: fix nil pointer error in Dockerfile.tmpl

[gcurtis]

Summary

Check that each stage is set in the plan before attempting to use its command. This fixes a nil pointer error that occurs when using an empty devbox.json in an unsupported language.

How was it tested?

Ran devbox plan in a Go project with an empty devbox.json.

[loreto]

LGTM

[ebauger]

I got this problem on the 0.0.4

$ devbox build
Error: template: Dockerfile.tmpl:44:26: executing "Dockerfile.tmpl" at <.StartStage.Image>: nil pointer evaluating *planner.Stage.Image
Usage:
  devbox build [<dir>] [flags]

Flags:
      --engine string   Engine used to build the container: 'docker', 'podman' (default "docker")
  -h, --help            help for build
      --no-cache        Do not use a cache

[loreto]

@ebauger We just released 0.0.5 and it includes this PR. Hopefully that fixes it!