test: fix multi-sudo dockerfile

[crazy-max]

relates to https://github.com/docker/build-push-action/actions/runs/12370695852/job/34525242942#step:6:314

#18 [linux/amd64 build 4/4] RUN sudo chown buildx. /log
#18 0.071 chown: unknown user buildx.
#18 ERROR: process "/bin/sh -c sudo chown buildx. /log" did not complete successfully: exit code: 1

#19 [linux/amd64->arm64 build 4/4] RUN sudo chown buildx. /log
#19 0.059 chown: unknown user buildx.
#19 ERROR: process "/bin/sh -c sudo chown buildx. /log" did not complete successfully: exit code: 1
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 > [linux/amd64 build 4/4] RUN sudo chown buildx. /log:
0.071 chown: unknown user buildx.
------
------
 > [linux/amd64->arm64 build 4/4] RUN sudo chown buildx. /log:
0.059 chown: unknown user buildx.
------
multi-sudo.Dockerfile:15
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  13 |     
  14 |     USER buildx
  15 | >>> RUN sudo chown buildx. /log
  16 |     USER root
  17 |     
--------------------
ERROR: failed to solve: process "/bin/sh -c sudo chown buildx. /log" did not complete successfully: exit code: 1

[crazy-max]

Getting this one in to unblock other PRs