[4.2] Dockerfile: blow out quay.io cache

[miabbott]

We want to pull in the v2020.1 version of rpm-ostree for the
exclude-packages support[0]. @jlebon was nice enough to rebuild
rpm-ostree with that support in F30, so blow out the cache to make
sure we pick it up.

[0] coreos/rpm-ostree#1980

[miabbott]

Successful RHCOS build using exclude-packages here - https://url.corp.redhat.com/219302d

[jlebon]

/approve
/lgtm

[openshift-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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[miabbott]

Weird, we have lgtm_acts_as_approve set for this repo - https://github.com/openshift/release/blob/master/core-services/prow/02_config/_plugins.yaml#L83-L86

[jlebon]

Ahh, there is no OWNERS on this branch, and there is also no .cci.jenkinsfile either. Though not sure how useful it'd be if it's building git master FCOS instead of RHCOS anyway. Let's just manually merge it since it's not actually doing any code changes anyway (and at least Travis CI is green).