fix(docker): remove bash-dependencies from build script

The build script for the backend relied on some bash specifics.
Since bash is not available in alpine based node docker images,
building failed. This commit replaces bash functions with the
native POSIX equivalents.

Signed-off-by: Erik Michelson <github@erik.michelson.eu>

backend/build.sh

@@ -1,13 +1,13 @@
-#!/usr/bin/env bash
+#!/bin/sh
 #
 # SPDX-FileCopyrightText: 2025 The HedgeDoc developers (see AUTHORS file)
 #
 # SPDX-License-Identifier: AGPL-3.0-only
 #
-set -euo pipefail
+set -eu
 
-_script_dir=$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)
-cd "${_script_dir}" || exit 1
+SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd)"
+cd "$SCRIPT_DIR" || exit 1
 
 if [ -d "dist" ]; then
   echo "🦔 > Clearing dist directory"
@@ -19,9 +19,10 @@ nest build
 
 if [ -d "dist/src/database/migrations" ]; then
   echo "🦔 > Removing TypeScript definitions from migrations folder"
-  rm -rf dist/src/database/migrations/*.d.ts 2>/dev/null || true
-  rm -rf dist/src/database/migrations/*.map 2>/dev/null || true
+  rm -f dist/src/database/migrations/*.d.ts 2>/dev/null || true
+  rm -f dist/src/database/migrations/*.map 2>/dev/null || true
 fi
 
 echo "🦔 > Done building backend"
 exit 0
+