fix: pin Alpine to 3.21.3 to resolve Docker manifest list issue

The release workflow failed with a Docker manifest error when trying to
create multi-platform manifests. The error message indicated that the
individual platform-specific images (e.g., newrelic/cli:v0.106.23-amd64)
were already manifest lists, preventing goreleaser from combining them
into the final multi-arch manifest.

Root cause:
- Alpine 3.21 (floating tag) introduced changes in manifest handling
- goreleaser v1.15.2 + Docker buildx interaction with multi-platform
  base images can result in platform-specific builds retaining manifest
  list metadata instead of being pure single-platform images

Fix:
- Pin Alpine to specific patch version 3.21.3
- This ensures consistent manifest structure and resolves the buildx
  manifest list propagation issue

Error from failed release:
⨯ release failed after 24s error=docker manifests: failed to publish
artifacts: failed to create newrelic/cli:v0.106.23: exit status 1:
docker.io/newrelic/cli:v0.106.23-amd64@sha256:... is a manifest list

Related: PR #1786 (Alpine upgrade from 3.20.3 to 3.21)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: pranav-new-relic

build/package/Dockerfile

@@ -1,4 +1,4 @@
-FROM alpine:3.21
+FROM alpine:3.21.3
 
 # Add the binary
 COPY newrelic /bin/newrelic