dataflow: update Dockerfile and modularize tests

[davidcavazos]

Description

Updating the Dockerfile, but while doing that I updated the test to make it e2e and actually test the image creation. There's been a lot of code duplication in tests so I tried to modularize common functions into their own file. I'll migrate other tests to use that module later on.

Checklist

• I have followed Sample Guidelines from AUTHORING_GUIDE.MD
• README is updated to include all relevant information
• Tests pass: nox -s py-3.6 (see Test Environment Setup)
• Lint pass: nox -s lint (see Test Environment Setup)
• These samples need a new API enabled in testing projects to pass (let us know which ones)
• These samples need a new/updated env vars in testing projects set to pass (let us know which ones)
• Please merge this PR for me once it is approved.
• This sample adds a new sample directory, and I updated the CODEOWNERS file with the codeowners for this sample

[davidcavazos]

It looks like tests are passing. It now waits until the job is running (typically 2-3 minutes), then waits 1 minute for data to come in, and cancels the job.

[davidcavazos]

@leahecole @dandhlee tests are passing, and comments addressed. If there are no other comments, this might be ready to merge :)

[leahecole]

Dan is OOO today but he'll look when he's back!

[dandhlee]

LGTM, just a minor comment below. Thank you!

[davidcavazos]

All tests are passing. I'll merge this for now to unblock writing other tests. If we want to revisit and remove the unused utility function, I'll do it in a separate PR.