Ensure the callback function in the dockerfileBuildDialog
directive is set, and therefore that it's ready to start a build,
before enabling the button to trigger start build.
Fixes: [Issue #203931642/](https://sentry.io/coreos/frontend-production/issues/203931642/)
There are a number of small state fixes in this PR, which result in the build dialog being properly reset state-wise each time it is displayed, thus fixing the state inconsistency.
Fixes#2106