Warn if docker buildx is not available

[dims]

Follow up to #104245 Let us warn folks with a better error message.

cc @m-Bilal

Signed-off-by: Davanum Srinivas davanum@gmail.com

What type of PR is this?

/kind cleanup

What this PR does / why we need it:

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

ensure_dockerized was not being used anywhere ( https://cs.k8s.io/?q=ensure_dockerized&i=nope&files=&excludeFiles=&repos=kubernetes/kubernetes,kubernetes/test-infra )

Does this PR introduce a user-facing change?

NONE

Additional documentation e.g., KEPs (Kubernetes Enhancement Proposals), usage docs, etc.:

[k8s-ci-robot]

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[dims]

/assign @saschagrunert

[saschagrunert]

/lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

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/retest

[dims]

whoops fixed shell check failure

In ./hack/lib/util.sh line 714:
  if docker buildx 2>&1 >/dev/null; then
                   ^--^ SC2069: To redirect stdout+stderr, 2>&1 must be last (or use '{ cmd > file; } 2>&1' to clarify).

For more information:
  https://www.shellcheck.net/wiki/SC2069 -- To redirect stdout+stderr, 2>&1 m...

[pacoxu]

/lgtm

[saschagrunert]

/test pull-kubernetes-conformance-kind-ipv6-parallel