create dockerfile to run echo service

[wenchenglu]

endpoints alpha doc will need to link to this dockerfile

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[theacodes]

This will actually break the flexible samples, as the presence of a Dockerfile will cause gcloud app deploy to complain about runtime not being set to custom.

We have two options:

• We could put the Dockerfile elsewhere or by a different name and tell the user to copy it.
• we can duplicate this sample to container_engine/endpoints.

[wenchenglu]

if I rename this to echo.Dockerfile, will this solve that problem?

By the way, like the idea of having a sample in container_engine/endpoints. But I am hesitating on causing code duplicacy. Let me know what you would prefer :)

[wenchenglu]

they don't need to copy it. we document in the user doc and ask them to run:
docker build -t image_path ./echo.Dockerfile

[theacodes]

I'd prefer Dockerfile.container-engine.

[wenchenglu]

done. in the same directory? not sure if this will pass the CI.

By the way, this sample will be used for both gce and gke.

[theacodes]

For the record @wlu2016 is a Googler and no CLA is needed.

[theacodes]

gevent shouldn't be needed.

[wenchenglu]

done ptal

[theacodes]

LGTM.