Bug: documentation for build_env is unclear and could use an example

[mikemccracken]

stacker version

v0.40.1

Describe the bug

the build_env section takes a yaml dictionary, which is used directly without any substitution or reference to the calling environment, to fill in the environment of the container process.

this isn't really clear from the doc.
that means that it is just syntactic sugar for export FOO=bar in the run section.

if there is another use case, we should explain and document it.

the current docs leave room to imagine that it does more than it does, e.g. maybe it expands variables from the calling environment or something, but it does not do that.

in either case, an example showing what the expected syntax is, and explaining that substitutions in this section still need to be provided as arguments via --substitution , will be useful.