gracefully handle rate-limits of snapshot.d.o

When building the kas-container in CI, the packages are fetched from
snapshot.d.o. As this service recently applied strict rate limits
(req/s), we need to retry multiple times (with exponential backoff) to
get reliable builds.

Signed-off-by: Felix Moessbauer <felix.moessbauer@siemens.com>
[Jan: fixed up typo]
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>

Dockerfile

@@ -42,6 +42,8 @@ RUN --mount=type=cache,target=/var/cache/apt,sharing=${CACHE_SHARING} \
         sed -i -e '/^URIs:/d' -e 's|^# http://snapshot\.|URIs: http://snapshot.|' \
             /etc/apt/sources.list.d/debian.sources; \
         echo 'Acquire::Check-Valid-Until "false";' > /etc/apt/apt.conf.d/use-snapshot.conf; \
+        echo 'Acquire::Retries "10";' >> /etc/apt/apt.conf.d/use-snapshot.conf; \
+        echo 'Acquire::Retries::Delay::Maximum "600";' >> /etc/apt/apt.conf.d/use-snapshot.conf; \
     fi && \
     apt-get update && \
     apt-get install -y locales && \