Allow overriding of the default response format for custom errors

[ianroberts]

What this PR does / why we need it:

Currently the example custom-errors back-end always defaults to responding with HTML error messages if the caller's Accept header does not specify otherwise. This is sub-optimal in cases where, for example, we are generating custom errors for an ingress that refers to a REST API endpoint whose default response format is e.g. JSON - in the absence of an Accept header, the default format of the error messages should be the same as the default format of the endpoint's own successful responses.

This PR modifies the example custom error generator so it can take the default response MIME type from an environment variable, instead of always using text/html. HTML is still the default if the variable is not set, to preserve backwards compatibility, but this makes it possible to set DEFAULT_RESPONSE_FORMAT=application/json in the deployment spec for the case described above to make a particular instance respond in JSON by default.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

How Has This Been Tested?

I am running a copy of this code in my own cluster successfully.

Checklist:

• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I've read the CONTRIBUTION guide
• I have added tests to cover my changes. N/A
• All new and existing tests passed. N/A - this is a change to example code that no tests depend on

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[k8s-ci-robot]

Hi @ianroberts. Thanks for your PR.

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[ianroberts]

CLA signed.

[rikatz]

/ok-to-test

[rikatz]

@ianroberts thanks for your PR!
As this gets merged, the image will need to be promoted and a new PR opened to point to the new image.

I'll post the sha of the new image here

[rikatz]

/retest

[rikatz]

@ianroberts can you please rebase over main? This problem is a bit well known, and was solved on past :)

Thanks!

[ianroberts]

Doesn't seem to have helped, sorry. Though I can't see how anything I've changed in my PR could cause it to fail this way, as I haven't touched any code in the ingress controller itself, only the custom errors image.

[rikatz]

hum, this may be my fault, as I just did some changes in image runner. Let me check here

[rikatz]

https://prow.k8s.io/view/gs/kubernetes-jenkins/pr-logs/pull/ingress-nginx/7457/pull-ingress-nginx-test/1423734457763893248

yeap, tried to reproduce and got the same error.

Let me fix this here

[rikatz]

@ianroberts please rebase again, now this should work :)

This was due to the migration to go v1.16 and some changes

[rikatz]

/retest

[rikatz]

no need to rebase
/lgtm
/approve
Thanks!

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: ianroberts, rikatz

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[rikatz]

So I though this image was published on gcr.k8s.io but it isn't so no further action seems to be required.

Thanks