Rewrite digests step to loop over architecture artifacts dynamically

[Copilot]

The manifest creation step hardcoded amd64 and arm64 digest outputs, causing failures for CUDA images that only support amd64 (empty arm64 digest passed to docker buildx imagetools create).

Changes

• Dynamic artifact discovery: Loop over all available .digest files instead of hardcoding architecture names
• New all output variable: Collect all digests into single space-separated variable
• Derive metadata from first digest: Extract registry, image, and tag from first discovered digest
• Simplified imagetools create: Use unpacked all variable instead of explicit architecture outputs
• Error handling: Added nullglob for safe glob expansion and validation that at least one digest exists

Applied to both base-manifest and eic-manifest jobs.

# Before
echo "amd64=$DIGEST_AMD64" >> $GITHUB_OUTPUT
echo "arm64=$DIGEST_ARM64" >> $GITHUB_OUTPUT
docker buildx imagetools create $TAG_ARGS \
  ${{ steps.digests.outputs.amd64 }} \
  ${{ steps.digests.outputs.arm64 }}

# After
shopt -s nullglob
for digest_file in /tmp/digests/${{ matrix.BUILD_IMAGE }}-*.digest; do
  # collect digests...
done
echo "all=$ALL_DIGESTS" >> $GITHUB_OUTPUT
docker buildx imagetools create $TAG_ARGS \
  ${{ steps.digests.outputs.all }}

---

💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more Copilot coding agent tips in the docs.