Add --branch to build images for a specific Gerrit branch

Introduce a new `--branch` option to the `build` script, enabling images
to be built against a Gerrit branch other than `master`. The branch is
forwarded to Docker as `--build-arg GERRIT_BRANCH=<branch>` and defaults
to `master` when not provided.

This makes it easier to build and deploy stable branches.

Bug: Issue 455813734
Change-Id: I0b3017519bfed4b62b27c89cbdbebdc68f5321e7

README.md

diff --git a/README.md b/README.md
index 0eac1a6..4f572ad 100644
--- a/README.md
+++ b/README.md
@@ -49,6 +49,15 @@
 ./build --gerrit-url https://example.com/gerrit.war
 ```
 
+The version of plugins and modules built into the images can be changed by providing
+the Gerrit branch. By default, the branch is set to `master`:
+```
+./build --branch stable-3.13
+```
+
+Note, if you do not include the `--gerrit-url` option, the image will automatically use
+the Gerrit release that corresponds to the selected `--branch`.
+
 The version of a health-check plugin built into the images can be changed by
 providing a download URL for a `.jar`-file containing the plugin:
 

build

diff --git a/build b/build
index 4458f1f..9202147 100755
--- a/build
+++ b/build
@@ -16,7 +16,7 @@
 
 usage() {
     me=`basename "$0"`
-    echo >&2 "Usage: $me [--help] [--tag TAG] [--no-cache] [--gerrit-url URL] [--base-image IMAGE] [--platform PLATFORM] [IMAGE]"
+    echo >&2 "Usage: $me [--help] [--tag TAG] [--no-cache] [--gerrit-url URL] [--base-image IMAGE] [--platform PLATFORM] [--branch GERRIT_BRANCH] [IMAGE]"
     exit 1
 }
 
@@ -61,6 +61,12 @@
     shift
     ;;
 
+  --branch)
+    shift
+    GERRIT_BRANCH=$1
+    shift
+    ;;
+
   *)
     break
   esac
@@ -71,6 +77,7 @@
 IMAGES=$(get_image_list)
 
 PLATFORM=${PLATFORM:-linux/amd64}  # Default value if PLATFORM is not set
+GERRIT_BRANCH=${GERRIT_BRANCH:-master} # Default value if GERRIT_BRANCH is not set
 DOCKER_BUILD_OPTS="--platform=$PLATFORM"
 test -n "$NO_CACHE" && DOCKER_BUILD_OPTS="$DOCKER_BUILD_OPTS --no-cache"
 
@@ -103,7 +110,7 @@
 }
 
 docker_build_gerrit_base(){
-    BUILD_ARGS="$BUILD_ARGS --build-arg TAG=$REV"
+    BUILD_ARGS="$BUILD_ARGS --build-arg TAG=$REV --build-arg GERRIT_BRANCH=$GERRIT_BRANCH"
     docker build \
         $DOCKER_BUILD_OPTS \
         $BUILD_ARGS \