A newer imagebuilder was deployed to OSBS which now respects .dockerignore during operations such as COPY. The .dockerignore in openshift/thanos must not include /vendor, as the container builds with -mod=vendor and is not allowed to fetch external dependencies.
Since the problem described in this bug report should be resolved in a recent advisory, it has been closed with a resolution of ERRATA. For information on the advisory (Moderate: OpenShift Container Platform 4.9.0 bug fix and security update), and where to find the updated files, follow the link below. If the solution does not work for you, open a new bug report. https://access.redhat.com/errata/RHSA-2021:3759