ci: fix ghcr.io container pushing
Attempt to fix following:
#11 ERROR: denied: permission_denied: The requested installation does not exist.
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> pushing ghcr.io/ynezz/buildworker-v3.5.0:master with docker:
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ERROR: denied: permission_denied: The requested installation does not exist.
Error: buildx failed with: ERROR: denied: permission_denied: The requested installation does not exist.
References: https://github.com/openwrt/buildbot/actions/runs/
4971701948/jobs/
8899889081#step:9:191
Signed-off-by: Petr Štetiar <ynezz@true.cz>