[BEAM-3827] Invoke Go Dataflow integration tests from post-commit

[herohde]

• Add a Python ValidatesContainer-style script to build the Go container around the integration tests. It uses the same bash logic for consistency, but ideally we'd have a better solution for building containers for integration tests shared across the SDKs.
• Add another Gradle build file to build the linux_amd64 Go integration test worker with a fixed name. This separation makes it easier to invoke the correct test driver from the script, because we don't have to figure out what the host platform and architecture is. It didn't seem possible to do this from a single Gradle build file.

Design notes: https://docs.google.com/document/d/1jy6EE7D4RjgfNV0FhD3rMsT1YKhnUfcHRZMAlC6ygXw/edit?usp=sharing

[herohde]

Run Go PostCommit

[herohde]

R: @alanmyrvold @jasonkuster

[herohde]

R: @lostluck @kennknowles

[kennknowles]

LGTM