fix(docker): support TZ environment variable (#5891)

Install tzdata in both published runtime variants and verify IANA timezone resolution during image builds.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

scripts/test_docker_runtime_timezone.sh

@@ -0,0 +1,24 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+ROOT_DIR=$(cd "$(dirname "${BASH_SOURCE[0]}")/.." && pwd)
+
+for dockerfile in Dockerfile Dockerfile.glibc; do
+  runtime_stage=$(
+    awk '
+      /^FROM[[:space:]]/ { stage = "" }
+      { stage = stage $0 ORS }
+      END { printf "%s", stage }
+    ' "$ROOT_DIR/$dockerfile"
+  )
+
+  if ! grep -Eq '^[[:space:]]*tzdata([[:space:]]*\\)?[[:space:]]*$' <<<"$runtime_stage"; then
+    echo "$dockerfile runtime stage must install tzdata" >&2
+    exit 1
+  fi
+
+  if ! grep -Fq 'test "$(TZ=Asia/Kolkata date +%z)" = "+0530"' <<<"$runtime_stage"; then
+    echo "$dockerfile runtime stage must verify IANA timezone resolution" >&2
+    exit 1
+  fi
+done