Fix Docker builds for nerves_system_br v1.28.0 and later

[fhunleth]

Fixes #997

[fhunleth]

@jjcarstens I think this PR works for nerves_system_br 1.28.x. At least it works on my one test. I'm getting this error with earlier systems and I think it's something else:

➜  nerves_system_bbb git:(v2.22.1) ✗ mix compile
==> nerves
Compiling 2 files (.ex)
Generated nerves app
==> nerves_system_bbb
Generated nerves_system_bbb app
|Elixir.Docker| Starting Build... (this may take a while)

** (Mix) The Nerves Docker build_runner encountered an error while building:

-----
Setting User
UID: 501
GID: 20
useradd: warning: the home directory /home/nerves already exists.
useradd: Not copying any file from skel directory into it.
Switching user
error: exec: "/nerves/env/platform/create-build.sh": stat /nerves/env/platform/create-build.sh: no such file or directory

-----

[fhunleth]

Now I can't reproduce the issue I was seeing. I upgraded Docker on my laptop since I had an old version, and I cleaned the build directory. Maybe it was the latter.

I'm going to mark this as ready for review since it seems to work.