Optionally add pull secret for private ECR

[philippthun]

While testing with a private ECR, we figured out that the ecr_helper.sh script does not create the necessary pull secret of type kubernetes.io/dockerconfigjson. We added an optional flag --push-and-pull to the helper script to support private ECRs.

[evankanderson]

/ok-to-test

I don't really have an easy way to validate this, but it looks reasonable.

[evankanderson]

/lgtm /approve

[AkihiroSuda]

LGTM (IANAM)

In long-term, I expect Knative Build to support kubernetes.io/dockerconfigjson for pushing as well https://github.com/knative/build/issues/389

[evankanderson]

/assign @ImJasonH

Since I can't approve

[philippthun]

@ImJasonH - As this PR has been reviewed and approved by @evankanderson, would it be possible to merge it? Or is something missing from your point of view?

[knative-prow-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: evankanderson, ImJasonH, philippthun

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[philippthun]

@evankanderson - I've fixed a linting error in the README.md (see diff: https://github.com/knative/build-templates/compare/6b7a3b26536caf40ed092554c98b3c1ffc7d05f3..90020a86d8187ce1cfa2885be99048ae19400d9b). Could you please have a look?

[evankanderson]

/lgtm

(Sorry, missed your comment)

[philippthun]

/retest