chore: add script to install yarn dependencies

* Use frozen lockfile in build for reproducible builds
* Do not install optional dependencies
* Suppress interactive prompts in build

scripts/yarn_install.sh

@@ -0,0 +1,42 @@
+#!/bin/bash
+#
+# Run "yarn install" with flags appropriate to the environment
+# (local development vs build system)
+#
+# Usage: yarn_install.sh [optional extra flags]
+
+set -euo pipefail
+
+PROJECT_ROOT=$(git rev-parse --show-toplevel)
+cd "$PROJECT_ROOT/site"
+
+yarn_flags=(
+  # Do not execute install scripts
+  # TODO: check if build works properly with this enabled
+  # --ignore-scripts
+
+  # Check if existing node_modules are valid
+  # TODO: determine if this is necessary
+  # --check-files
+
+  # Do not install optional dependencies
+  --ignore-optional
+)
+
+if [ -n "${CI:-}" ]; then
+  yarn_flags+=(
+    # Install dependencies from lockfile, ensuring builds are fully
+    # reproducible
+    --frozen-lockfile
+    # Suppress progress information
+    --silent
+    # Disable interactive prompts for build
+    --non-interactive
+  )
+fi
+
+# Append whatever is specified on the command line
+yarn_flags+=("$@")
+
+echo "+ yarn install ${yarn_flags[*]}"
+yarn install "${yarn_flags[@]}"