Difficult using buildcontainers on Linux

[eriknordmark]

First of all needed to fix some things for Linux - see #15

But I don't understand the easy way to use buildcontainers to do make proto; the above PR has a manual way to do it.

Do we needs some make docker-proto Makefile target to do this?
Hopefully we can do it without all of the complexity we have in lf-edge/eve/Makefile.

[rene]

We now support make proto-container.