Add ShellCheck CI and fix reported issues

Add a GitHub Actions workflow using differential-shellcheck to check
shell scripts on pull requests. Fix the issues it found:

- build.sh: replace indirect $? checks with direct `if !` form (SC2181)
- src/bash_functions.sh: separate export from assignment to avoid masking
  return values (SC2155)

Author: lightswitch05

.github/workflows/shellcheck.yml

@@ -0,0 +1,18 @@
+name: ShellCheck
+on:
+  pull_request:
+    types: [opened, synchronize, reopened, ready_for_review]
+
+permissions:
+  contents: read
+
+jobs:
+  shellcheck:
+    if: github.event.pull_request.draft == false
+    runs-on: ubuntu-latest
+    steps:
+    - name: Checkout repository
+      uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd #v6.0.2
+
+    - name: Run ShellCheck
+      run: find . -name "*.sh" -exec shellcheck {} +

build.sh

@@ -27,8 +27,7 @@ WEB_FORK="pi-hole"
 PADD_FORK="pi-hole"
 
 # Check if buildx is installed
-docker buildx version >/dev/null 2>&1
-if [ $? -ne 0 ]; then
+if ! docker buildx version >/dev/null 2>&1; then
     echo "Error: Docker buildx is required to build this image. For installation instructions, see:"
     echo "       https://github.com/docker/buildx#installing"
     exit 1
@@ -151,10 +150,7 @@ DOCKER_BUILD_CMD+=("${BUILD_ARGS[@]}")
 
 # Execute the docker build command
 echo "Executing command: ${DOCKER_BUILD_CMD[*]}"
-"${DOCKER_BUILD_CMD[@]}"
-
-# Check exit code of previous command
-if [ $? -ne 0 ]; then
+if ! "${DOCKER_BUILD_CMD[@]}"; then
     echo ""
     echo "!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!"
     echo "!! ERROR: Docker build failed, please review logs above !!"

src/bash_functions.sh

@@ -191,7 +191,8 @@ migrate_v5_configs() {
 setup_web_password() {
     if [ -z "${FTLCONF_webserver_api_password+x}" ] && [ -n "${WEBPASSWORD_FILE}" ] && [ -r "/run/secrets/${WEBPASSWORD_FILE}" ]; then
         echo "  [i] Setting FTLCONF_webserver_api_password from file"
-        export FTLCONF_webserver_api_password=$(<"/run/secrets/${WEBPASSWORD_FILE}")
+        FTLCONF_webserver_api_password=$(<"/run/secrets/${WEBPASSWORD_FILE}")
+        export FTLCONF_webserver_api_password
     fi
 
     # If FTLCONF_webserver_api_password is not set