ci(msan): retry transient network failures while building the MSan image

The test-msan lane dies whenever the buildx layer cache misses and the
builder's resolver hiccups. PR-CI runs 33816056229 and 33818108315 both
failed in "Build MSan image (cached layers)" with:

    #7 26.77   Could not resolve 'apt.llvm.org'
    #7 27.34 W: Failed to fetch http://apt.llvm.org/noble/dists/\
        llvm-toolchain-noble-22/InRelease  Could not resolve 'apt.llvm.org'
    ERROR: failed to build: failed to solve: process "/bin/sh -c apt-get \
        update && ..." did not complete successfully

The wget of the signing key from that SAME host had succeeded seconds
earlier in the same layer, so this is a transient DNS failure inside
buildkit, not a wrong URL or a dead mirror. 2 of the last 15 PR runs.

Every network-touching step in Dockerfile.msan now runs under a small
POSIX-sh helper written into the image before the first layer that needs
it: /usr/local/bin/retry makes 5 attempts with growing backoff
(5s/10s/20s/40s) and still exits non-zero once they are exhausted, so a
genuine breakage keeps failing the build instead of being swallowed.
Wrapped: both apt-get update/install pairs (also given
-o Acquire::Retries=3), the wget of the LLVM signing key, and the
llvm-project and zlib clones. Two shapes needed care:

- the key is fetched with `wget -O <file>` instead of `-qO- > <file>`,
  because a redirect is opened once for all attempts and a partial write
  from a failed attempt would be prepended to a later successful one;
- each clone clears its destination first, or a half-finished clone makes
  every later attempt fail on "destination path already exists".

The pinned base digest, the pinned llvmorg-22.1.0 / v1.3.1 tags,
--no-install-recommends and the layer structure are unchanged; on a
healthy network the image is byte-for-byte the same work as before.

Verified on Colima:
- `docker buildx build --check -f test-infrastructure/Dockerfile.msan
  test-infrastructure/` -> "Check complete, no warnings found."
- scratch image = FROM + helper + the real apt.llvm.org layer, built
  --no-cache: layer completes, clang-22/llvm-22 installed.
- scratch image where the wrapped command fails twice then succeeds:
  attempts 1 and 2 logged with 5s/10s backoff, attempt 3 succeeds,
  build exit 0.
- scratch image where the wrapped command always fails: 5 attempts,
  "retry: giving up after 5 attempts", retry_exit=1, build exit 1.

The full MSan image was deliberately not built locally (it compiles
libc++ with MSan); CI builds it on the branch.

Signed-off-by: Martin Vogel <martin.vogel.tech@gmail.com>

Author: DeusData

test-infrastructure/Dockerfile.msan

@@ -16,18 +16,47 @@
 # Same pinned base as the primary test image — bump deliberately, never to a tag.
 FROM ubuntu:noble@sha256:4fbb8e6a8395de5a7550b33509421a2bafbc0aab6c06ba2cef9ebffbc7092d90
 
+# WHY every network step below is wrapped: on a cache miss the builder's
+# resolver hiccups and takes the whole lane down. PR-CI runs 33816056229 and
+# 33818108315 both died with `Could not resolve 'apt.llvm.org'` inside the
+# clang layer — seconds AFTER a wget from that same host had succeeded in the
+# same layer, i.e. a transient buildkit-side DNS failure, not a wrong URL.
+# retry: 5 attempts with growing backoff, and it still exits non-zero after
+# the last one, so a genuine breakage keeps failing the build.
+RUN printf '%s\n' \
+    '#!/bin/sh' \
+    '# retry <cmd> [args...]: 5 attempts, sleeping 5s/10s/20s/40s between them.' \
+    'attempt=1; delay=5' \
+    'until "$@"; do' \
+    '  if [ "$attempt" -ge 5 ]; then' \
+    '    echo "retry: giving up after $attempt attempts: $*" >&2' \
+    '    exit 1' \
+    '  fi' \
+    '  echo "retry: attempt $attempt failed, sleeping ${delay}s before retrying: $*" >&2' \
+    '  sleep "$delay"' \
+    '  attempt=$((attempt + 1)); delay=$((delay * 2))' \
+    'done' \
+    > /usr/local/bin/retry \
+    && chmod 0755 /usr/local/bin/retry
+
 # clang 22 from apt.llvm.org, matching the diag and analyzer lanes. Noble's
 # default is clang 18 — four majors behind everything else here, which is both
 # an inconsistency and a bad vantage point for debugging sanitizer behaviour.
-RUN apt-get update && apt-get install -y --no-install-recommends wget gnupg ca-certificates \
-    && wget -qO- https://apt.llvm.org/llvm-snapshot.gpg.key > /etc/apt/trusted.gpg.d/apt.llvm.org.asc \
+# The key is fetched with `wget -O <file>` rather than `-qO- > <file>`: under
+# retry a redirect is opened once for all attempts, so a partial write from a
+# failed attempt would be prepended to the output of a later successful one.
+RUN retry apt-get -o Acquire::Retries=3 update \
+    && retry apt-get -o Acquire::Retries=3 install -y --no-install-recommends wget gnupg ca-certificates \
+    && retry wget -q -O /etc/apt/trusted.gpg.d/apt.llvm.org.asc https://apt.llvm.org/llvm-snapshot.gpg.key \
     && echo "deb http://apt.llvm.org/noble/ llvm-toolchain-noble-22 main" > /etc/apt/sources.list.d/llvm-22.list \
-    && apt-get update && apt-get install -y --no-install-recommends \
+    && retry apt-get -o Acquire::Retries=3 update \
+    && retry apt-get -o Acquire::Retries=3 install -y --no-install-recommends \
        clang-22 libclang-rt-22-dev llvm-22 \
     && ln -sf /usr/bin/clang-22 /usr/bin/clang \
     && ln -sf /usr/bin/clang++-22 /usr/bin/clang++
 
-RUN apt-get update && apt-get install -y --no-install-recommends \
+RUN retry apt-get -o Acquire::Retries=3 update \
+    && retry apt-get -o Acquire::Retries=3 install -y --no-install-recommends \
     cmake \
     ninja-build \
     make \
@@ -40,9 +69,10 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
     && rm -rf /var/lib/apt/lists/*
 
 # libc++ + libc++abi + libunwind with MemoryWithOrigins, at the SAME major as
-# the compiler above — a runtimes build must match its clang.
-RUN git clone --depth 1 --branch llvmorg-22.1.0 \
-        https://github.com/llvm/llvm-project.git /tmp/llvm-project \
+# the compiler above — a runtimes build must match its clang. The clone clears
+# its destination first: a half-finished clone would make every later attempt
+# fail on "destination path already exists".
+RUN retry sh -c 'rm -rf /tmp/llvm-project && git clone --depth 1 --branch llvmorg-22.1.0 https://github.com/llvm/llvm-project.git /tmp/llvm-project' \
     && cmake -G Ninja -S /tmp/llvm-project/runtimes -B /tmp/llvm-msan \
         -DCMAKE_BUILD_TYPE=Release \
         -DCMAKE_C_COMPILER=clang \
@@ -59,7 +89,7 @@ RUN git clone --depth 1 --branch llvmorg-22.1.0 \
     && rm -rf /tmp/llvm-project /tmp/llvm-msan
 
 # zlib with MSan (static, so the runner needs no runtime path for it).
-RUN git clone --depth 1 --branch v1.3.1 https://github.com/madler/zlib.git /tmp/zlib \
+RUN retry sh -c 'rm -rf /tmp/zlib && git clone --depth 1 --branch v1.3.1 https://github.com/madler/zlib.git /tmp/zlib' \
     && cd /tmp/zlib \
     && CC=clang CFLAGS="-fsanitize=memory -fsanitize-memory-track-origins=2 -fno-omit-frame-pointer -O2" \
        ./configure --prefix=/opt/msan --static \