avoid docker slices for multiple env's

[edannenberg]

avoid docker slices for multiple env's

[edannenberg]

As max layers are limited probably a good idea, also moved the ENV command further down for possible cache leverage on rebuild.

[edannenberg]

Of course I missed the = which is required for multi assignments but not for single. ><
One downside to be aware of is that prev. defined ENV are not available inside a single ENV instruction:

# works
ENV foo $bar
ENV meh $foo

# doesn't
ENV foo=$bar \
    meh=$foo

One last thing I currently can't wrap my head around is this 3bd09c7

If the default is set it seems to override the actual value, removing the default fixes the issue. But the same thing works for DEF_BUILDER_CHOST down a few lines. Also fairly sure this worked before refactoring to multi assignment. Not sure what I'm overlooking here, time for some sleep.