refactor: streamline Docker build process in CI configuration and documentation

Updated the CI workflow to utilize a script for generating Docker build variable overrides, simplifying the build commands for Go and Web images. Adjusted the README to reflect the new build process and clarify the usage of environment variables, enhancing overall documentation clarity.

deploy/docker/write-bake-override.sh

@@ -0,0 +1,31 @@
+#!/usr/bin/env sh
+# Writes docker-bake.override.hcl for CI (buildx without --var support).
+set -eu
+OUT="${1:-docker-bake.override.hcl}"
+REGISTRY="${REGISTRY:?REGISTRY required}"
+IMAGE_TAG="${IMAGE_TAG:-latest}"
+SHORT_SHA="${SHORT_SHA:-dev}"
+SHA_FULL="${SHA_FULL:-}"
+CACHE_REF_GO="${CACHE_REF_GO:-}"
+CACHE_REF_WEB="${CACHE_REF_WEB:-}"
+cat >"$OUT" <<EOF
+# Generated by deploy/docker/write-bake-override.sh — do not commit.
+variable "REGISTRY" {
+  default = "${REGISTRY}"
+}
+variable "IMAGE_TAG" {
+  default = "${IMAGE_TAG}"
+}
+variable "SHORT_SHA" {
+  default = "${SHORT_SHA}"
+}
+variable "SHA_FULL" {
+  default = "${SHA_FULL}"
+}
+variable "CACHE_REF_GO" {
+  default = "${CACHE_REF_GO}"
+}
+variable "CACHE_REF_WEB" {
+  default = "${CACHE_REF_WEB}"
+}
+EOF