A label with a value of only numbers causes unmarshal error

[johnmaguire]

We've been using a Git short hash as a label to identify builds, like so:

    labels:
      com.duo.access-gateway.builder.version: 336fb8d4
      com.duo.access-gateway.code.version: v1.4.3
      com.duo.access-gateway.built: Tue, 11 Apr 2017 13:02:06 -0000

This has worked great, until today, when I tried to call docker-compose up on a file with the following labels:

    labels:
      com.duo.access-gateway.builder.version: 89526235
      com.duo.access-gateway.code.version: v1.4.4-beta0
      com.duo.access-gateway.built: Fri, 02 Jun 2017 14:38:41 -0000

The following error was returned:

ERROR: for access-gateway Cannot create container for service access-gateway: json: cannot unmarshal number into Go value of type string

It took me a while to track this down, but it's due to com.duo.access-gateway.builder.version being all numbers. I'm going to quote our values now, but this doesn't seem like it should be an issue -- or if it should be an issue, perhaps a more helpful error message could be provided.

[shin-]

That's kind of a meta issue of YAML as a language and implicit type-coercion, and I don't think there's a good way for us to go around that. On the other hand, I agree that we have the ability to catch this earlier and provide better feedback for the user when running into this issue.