Kotlin examples for configuring bootBuildImage's environment use wrong map configuration syntax

[wilkinsona]

We document setting some environment variables but the configuration is untested. @sdeleuze thinks the syntax may be wrong so this could turn into a documentation issue.

[jnizet]

I think I can take care of this

[wilkinsona]

Thanks, @jnizet, but I already have a fix lined up for this one.