chore(deps): docker: bump golang

Bumps the all-docker group with 1 update in the /agent directory: golang.


Updates `golang` from 1.26.3-bookworm to 1.26.4-bookworm

---
updated-dependencies:
- dependency-name: golang
  dependency-version: 1.26.4-bookworm
  dependency-type: direct:production
  update-type: version-update:semver-patch
  dependency-group: all-docker
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

agent/Dockerfile

@@ -4,7 +4,7 @@ ARG GH_VERSION=2.93.0
 FROM --platform=$TARGETPLATFORM jdxcode/mise:latest AS mise
 
 # Build gh with a patched Go toolchain; upstream packages can lag Go CVE fixes.
-FROM --platform=$TARGETPLATFORM golang:1.26.3-bookworm AS gh-builder
+FROM --platform=$TARGETPLATFORM golang:1.26.4-bookworm AS gh-builder
 ARG GH_VERSION
 RUN GOPROXY=direct GOBIN=/out go install "github.com/cli/cli/v2/cmd/gh@v${GH_VERSION}"