ci(docker): retry buildx setup on transient Docker Hub failures

The Docker Build, Test, and Publish workflow fails when
docker/setup-buildx-action can't pull the moby/buildkit:buildx-stable-1
image from Docker Hub. The failure happens during builder bootstrap at
the auth token exchange — a transient network blip (connection reset,
read timeout, rate limiting) that self-resolves on re-run.

Recent failure (run 30449230291, merge job):
  read tcp 10.1.0.171:45666->104.18.43.178:443: read: connection reset by peer

This has hit us before and will again — it's the same class of
transient Docker Hub flake that the merge job already retries for
imagetools create. But buildx setup had no retry, so a single network
hiccup killed the entire job (build, publish, or merge) even though
nothing was wrong with the code or the image.

Fix: wrap each of the 3 buildx setup steps (build, publish, merge jobs)
with continue-on-error + a conditional retry step. The maintained action
is preserved as-is — we just give it a second attempt if the first
fails. The action generates a unique builder name per invocation, so the
retry never collides with the failed first attempt. The second attempt
has no continue-on-error, so genuine persistent failures still fail the
job.

The docker/setup-buildx-action maintainer has explicitly said retry
belongs at the workflow level, not inside the action [1], and other
repos use this same continue-on-error pattern for this exact issue [2].

[1] docker/setup-buildx-action#510
[2] joshjhall/containers#688, ethpandaops/eth-client-docker-image-builder#391

.github/workflows/docker.yml

@@ -53,7 +53,19 @@ jobs:
       - name: Checkout code
         uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
 
+      # Retry once on transient Docker Hub / buildkit pull failures
+      # (connection reset, auth token timeout, rate limiting). The action
+      # generates a unique builder name per invocation so the retry doesn't
+      # collide with the failed first attempt. A genuine persistent failure
+      # still fails the job — only the first attempt has continue-on-error.
+      # Refs: docker/setup-buildx-action#510
       - name: Set up Docker Buildx
+        id: buildx
+        continue-on-error: true
+        uses: docker/setup-buildx-action@8d2750c68a42422c14e847fe6c8ac0403b4cbd6f # v3
+
+      - name: Set up Docker Buildx (retry)
+        if: steps.buildx.outcome == 'failure'
         uses: docker/setup-buildx-action@8d2750c68a42422c14e847fe6c8ac0403b4cbd6f # v3
 
       # Build once, load into the local daemon for testing.  Cached
@@ -146,7 +158,15 @@ jobs:
       - name: Checkout trusted source
         uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
 
+      # Retry once on transient Docker Hub / buildkit pull failures.
+      # See build job for rationale; same pattern.
       - name: Set up Docker Buildx
+        id: buildx
+        continue-on-error: true
+        uses: docker/setup-buildx-action@8d2750c68a42422c14e847fe6c8ac0403b4cbd6f # v3
+
+      - name: Set up Docker Buildx (retry)
+        if: steps.buildx.outcome == 'failure'
         uses: docker/setup-buildx-action@8d2750c68a42422c14e847fe6c8ac0403b4cbd6f # v3
 
       - name: Log in to Docker Hub
@@ -208,7 +228,15 @@ jobs:
           pattern: digest-*
           merge-multiple: true
 
+      # Retry once on transient Docker Hub / buildkit pull failures.
+      # See build job for rationale; same pattern.
       - name: Set up Docker Buildx
+        id: buildx
+        continue-on-error: true
+        uses: docker/setup-buildx-action@8d2750c68a42422c14e847fe6c8ac0403b4cbd6f # v3
+
+      - name: Set up Docker Buildx (retry)
+        if: steps.buildx.outcome == 'failure'
         uses: docker/setup-buildx-action@8d2750c68a42422c14e847fe6c8ac0403b4cbd6f # v3
 
       - name: Log in to Docker Hub