Bump spiffe/spire-agent from 1.15.0 to 1.15.1 in /docker (#54)

Bumps spiffe/spire-agent from 1.15.0 to 1.15.1.

---
updated-dependencies:
- dependency-name: spiffe/spire-agent
  dependency-version: 1.15.1
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

docker/Dockerfile.backend-spiffe

@@ -1,7 +1,7 @@
 # Backend with SPIFFE Agent Sidecar
 # Runs the backend inside the agent container for proper workload attestation
 
-FROM ghcr.io/spiffe/spire-agent:1.15.0 AS spire-agent
+FROM ghcr.io/spiffe/spire-agent:1.15.1 AS spire-agent
 FROM ghcr.io/spiffe/spire-server:1.15.0 AS spire-server
 
 # Build backend

docker/Dockerfile.fly

@@ -48,7 +48,7 @@ RUN ./palette-indexer -content /content -out /app/palette.db && \
 # ============================================
 # Stage 3: Get SPIRE binaries
 # ============================================
-FROM ghcr.io/spiffe/spire-agent:1.15.0 AS spire-agent
+FROM ghcr.io/spiffe/spire-agent:1.15.1 AS spire-agent
 
 # ============================================
 # Stage 4: Production Image with SPIRE Agent