tests: retry flaky apk-add-over-custom-dns step in dns.bats

"apk add bind-tools" inside a nested container, fetched via a live
custom DNS server (1.0.0.1) and external package repo, intermittently
fails ("Permission denied" from apk/wget) due to transient network/DNS
blips unrelated to sysbox -- reproducing this exact command manually
right after a failure succeeds cleanly. Wrap it with the existing
retry_run helper (already used elsewhere in the suite for this same
class of flakiness) across all 10 occurrences of the pattern in this
file.

Signed-off-by: Cesar Talledo <cesar.talledo@docker.com>