Makes jibExportDockerContext.targetDir configurable from build.gradle.

[coollog]

This makes jibExportDockerContext configurable like:

build.gradle:

jibExportDockerContext {
    targetDir = 'my-dir'
}

Whereas before it could only be configured from a command line property --jib.dockerDir

[TadCordle]

Should probably add this to the changelog

[patflynn]

@loosebazooka is it gradle custom to allow the user to place build outputs outside of the build directory?

[coollog]

Filed #493 to not allow exporting outside of build output directory.

[patflynn]

@coollog FWIW I'm not strongly opinionated about whether these files should even be considered build outputs. Just asking the question.

[coollog]

Yea we'll have exportDockerContext match whatever we decide for the tarball export feature.