Add warnings/error for image not specified

[janetkuo]

Follow-up #79 (comment):

image is a required field in Kubernetes and without it the generated file can't be used.

From compose spec, v1 doesn't allow image being specified with build, and v2 allows you to do that but the image will be built from build.

We can print an error when we see build in the compose file. Would love to hear more thoughts.

[surajssd]

With Kubernetes:

• image is must, until we come up with a build story

With OpenShift:

• If build given we can create buildconfig
• if image given well and good

cc: @kadel

[ngtuna]

Currently there is an unsupported key message if we see build in the compose file.
@surajssd : while working on the issue #92 of having image and build in the same compose service, you can also add a check for required fields like image

[ngtuna]

Closed via #195