docker: bump golang from 1.26.5-alpine to 1.27.0-alpine

[dependabot]

Bumps golang from 1.26.5-alpine to 1.27.0-alpine.

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[bgentry]

🤖 Codex review: Approved after dependency security and compatibility review.

Upgrade

• golang development image: 1.26.5-alpine → 1.27.0-alpine
• Reviewed head: cb2df5c4f26cb19104d244d850a2ad73d56c05f0

Security review

• Verified the exact digest-pinned Docker Hub indexes, platform manifests,
  embedded SLSA provenance, SPDX SBOMs, official Go archive hashes, generated
  Dockerfiles, image configuration, and package inventories. The new index is
  sha256:4c9fe60190a2a3350ddc51de80d0224b8a6698d12bdfc999fee45ea9d6c46dbc.
• Provenance resolves to canonical docker-library/golang source commit
  7aa947b3fda4de2c123815ab1873e50bc810569d; all Go archive hashes match
  official metadata. Alpine 3.24 base digests, supported platforms, entrypoint,
  user, workdir, and installed Alpine package inventory are unchanged.
• This is security-bearing: Go 1.27 incorporates the ten security fixes first
  released in Go 1.26.6, while the old image contains Go 1.26.5. No new hook,
  downloader, telemetry, credential access, or runtime behavior appears in the
  image recipe.

Compatibility verification

• docker compose -f docker-compose.dev.yaml config --quiet — passed
• make test under local Go 1.27.0 — passed across all three Go modules
• GitHub's full Go 1.27 matrix, lint, JavaScript, release, and public image
  checks are green.
• The exact digest could not be run locally because no Docker daemon is
  available. Platform manifests, image configuration, SBOM/provenance, and
  the Go 1.27 test matrix provide the compensating evidence.
• The River Pro image failure is the queue-wide Dependabot AWS credential
  failure and occurs before a project build.

Residual risk

• Low: embedded provenance is digest-anchored but not independently Cosign
  signed and declares the build non-reproducible; arm/v6 lacks an SPDX SBOM.
  The pre-existing compose command still installs river@latest. Go 1.27.1 is
  now available, so this reviewed image is one non-security patch behind.