Refactor build of docker images

[aledbf]

What this PR does / why we need it:

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)

Which issue/s this PR fixes

This PR removes the use of docker buildx from most of the makefile tasks. This is unfortunate but trying to run e2e tests in an s390x VM I found out the latest docker version is 18.06.
There only two places where we really need to use buildx, the main makefile when we release a version and building nginx itself. In the nginx image, we also had to remove luarocks due to core dumps generated running uarocks install`.

How Has This Been Tested?

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.
• All new and existing tests passed.

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: aledbf

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• OWNERS [aledbf]

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[aledbf]

/test pull-ingress-nginx-e2e-1-15

[aledbf]

/test pull-ingress-nginx-e2e-1-18

[aledbf]

/retest

[aledbf]

/retest

[aledbf]

/retest

[aledbf]

/retest

[k8s-ci-robot]

@aledbf: The following test failed, say /retest to rerun all failed tests:

Test name	Commit	Details	Rerun command
pull-ingress-nginx-e2e-1-15	67c962b	link	/test pull-ingress-nginx-e2e-1-15

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[aledbf]

/retest