Bigtable autoscaler: Add Dockerfile

[mattayes]

In which file did you encounter the issue?

N/A

Did you change the file? If so, how?

N/A

Describe the issue

A Dockerfile for the Bigtable autoscaling would be nice. Happy to submit the PR!

[andrewferlitsch]

@crwilcox Is there a plan for this enhancement?

[mattayes]

@andrewferlitsch Waiting on the 👍, happy to do the work once folks think it's a good idea.