Add debugging utils and comments to Fuchsia scripts

This should help when debugging a failure in the Fuchsia build in CI.

src/ci/docker/host-x86_64/x86_64-gnu-integration/Dockerfile

@@ -1,3 +1,6 @@
+# This job builds Fuchsia. See docs at XXX TODO and the build-fuchsia.sh script
+# in this directory for more details.
+
 FROM ubuntu:22.04
 
 ARG DEBIAN_FRONTEND=noninteractive

src/ci/docker/host-x86_64/x86_64-gnu-integration/build-fuchsia.sh

@@ -2,50 +2,72 @@
 
 # Downloads and builds the Fuchsia operating system using a toolchain installed
 # in $RUST_INSTALL_DIR.
+#
+# You may run this script locally using Docker with the following command:
+#
+# $ src/ci/docker/run.sh x86_64-gnu-integration
 
 set -euf -o pipefail
 
-INTEGRATION_SHA=1011e3298775ee7cdf6f6dc73e808d6a86e33bd6
+# Set this variable to disable updating the Fuchsia checkout. This is useful for
+# making local changes. You can find the Fuchsia checkout in `obj/fuchsia` in
+# your local checkout after running this job for the first time.
+KEEP_CHECKOUT=
+
+# Any upstream refs that should be cherry-picked. This can be used to include
+# Gerrit changes from https://fxrev.dev during development (click the "Download"
+# button on a changelist to see the cherry pick ref). Example:
+# PICK_REFS=(refs/changes/71/1054071/2 refs/changes/74/1054574/2)
 PICK_REFS=()
 
+# The commit hash of integration.git to check out (this controls the commit hash
+# of fuchsia.git too).
+INTEGRATION_SHA=1011e3298775ee7cdf6f6dc73e808d6a86e33bd6
+
 checkout=fuchsia
 jiri=.jiri_root/bin/jiri
 
 set -x
 
-# This script will:
-# - create a directory named "fuchsia" if it does not exist
-# - download "jiri" to "fuchsia/.jiri_root/bin"
-curl -s "https://fuchsia.googlesource.com/jiri/+/HEAD/scripts/bootstrap_jiri?format=TEXT" \
-    | base64 --decode \
-    | bash -s $checkout
-
-cd $checkout
-
-$jiri init \
-    -partial=true \
-    -analytics-opt=false \
-    .
-
-$jiri import \
-    -name=integration \
-    -revision=$INTEGRATION_SHA \
-    -overwrite=true \
-    flower \
-    "https://fuchsia.googlesource.com/integration"
-
-if [ -d ".git" ]; then
-    # Wipe out any local changes if we're reusing a checkout.
-    git checkout --force JIRI_HEAD
-fi
+if [ -z "$KEEP_CHECKOUT" ]; then
+    # This script will:
+    # - create a directory named "fuchsia" if it does not exist
+    # - download "jiri" to "fuchsia/.jiri_root/bin"
+    curl -s "https://fuchsia.googlesource.com/jiri/+/HEAD/scripts/bootstrap_jiri?format=TEXT" \
+        | base64 --decode \
+        | bash -s $checkout
 
-$jiri update -autoupdate=false
+    cd $checkout
 
-echo integration commit = $(git -C integration rev-parse HEAD)
+    $jiri init \
+        -partial=true \
+        -analytics-opt=false \
+        .
 
-for git_ref in "${PICK_REFS[@]}"; do
-    git fetch https://fuchsia.googlesource.com/fuchsia $git_ref
-    git cherry-pick --no-commit FETCH_HEAD
-done
+    $jiri import \
+        -name=integration \
+        -revision=$INTEGRATION_SHA \
+        -overwrite=true \
+        flower \
+        "https://fuchsia.googlesource.com/integration"
+
+    if [ -d ".git" ]; then
+        # Wipe out any local changes if we're reusing a checkout.
+        git checkout --force JIRI_HEAD
+    fi
+
+    $jiri update -autoupdate=false
+
+    echo integration commit = $(git -C integration rev-parse HEAD)
+
+    for git_ref in "${PICK_REFS[@]}"; do
+        git fetch https://fuchsia.googlesource.com/fuchsia $git_ref
+        git cherry-pick --no-commit FETCH_HEAD
+    done
+else
+    cd $checkout
+fi
 
+# Run the script inside the Fuchsia checkout responsible for building Fuchsia.
+# You can change arguments to the build by modifying this script.
 bash scripts/rust/build_fuchsia_from_rust_ci.sh