feat: add Docker setup exposing app on port 7876

- Dockerfile: php:apache base, installs PHP extensions + Composer,
  configures Apache to listen on port 7876, bundles app code,
  runs composer install at build time
- docker-compose.yml: web service (port 7876) + MariaDB LTS with
  healthcheck; writable volume for DB persistence
- docker/entrypoint.sh: generates .env from environment variables
  at container start, fixes runtime permissions
- docker/php.ini: tuned PHP limits (512M memory, 32M upload)
- .dockerignore: excludes .git, vendor, logs, cache, old docker dir
- .env.example: template for required environment variables

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

docker/entrypoint.sh

@@ -0,0 +1,44 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+ENV_FILE="/var/www/html/.env"
+
+upsert_kv() {
+  local key="$1" val="$2"
+  if grep -qE "^${key}=" "$ENV_FILE" 2>/dev/null; then
+    sed -i "s#^${key}=.*#${key}=${val}#" "$ENV_FILE"
+  else
+    printf "%s=%s\n" "$key" "$val" >> "$ENV_FILE"
+  fi
+}
+
+# Bootstrap .env from template if not present
+if [ ! -f "$ENV_FILE" ]; then
+  if [ -f /var/www/html/env.example.txt ]; then
+    cp /var/www/html/env.example.txt "$ENV_FILE"
+  else
+    touch "$ENV_FILE"
+  fi
+fi
+
+# Inject database and app config from environment variables
+upsert_kv "DB_HOST"     "${DB_HOST:-db}"
+upsert_kv "DB_PORT"     "${DB_PORT:-3306}"
+upsert_kv "DB_DATABASE" "${DB_DATABASE:-domain_monitor}"
+upsert_kv "DB_USERNAME" "${DB_USERNAME:-domain_monitor}"
+upsert_kv "DB_PASSWORD" "${DB_PASSWORD:-}"
+upsert_kv "APP_ENV"     "${APP_ENV:-production}"
+
+[ -n "${APP_ENCRYPTION_KEY:-}" ] && upsert_kv "APP_ENCRYPTION_KEY" "$APP_ENCRYPTION_KEY"
+[ -n "${SESSION_LIFETIME:-}" ]   && upsert_kv "SESSION_LIFETIME"   "$SESSION_LIFETIME"
+
+# Ownership & permissions on runtime-writable paths
+chown www-data:www-data "$ENV_FILE"
+chmod 660 "$ENV_FILE"
+
+for d in logs cache public/assets/uploads; do
+  dir="/var/www/html/$d"
+  [ -d "$dir" ] && chown -R www-data:www-data "$dir" && chmod -R 775 "$dir"
+done
+
+exec "$@"