Check if tables exist before installing

This works better than checking just the database, because it could be
that an empty database has already been created (with e.g. Docker
Compose).

TABLES_EXIST should be empty if no tables, or will return a 1
for *each* table that exists.

root/entrypoint.sh

@@ -76,6 +76,12 @@ provisioner() {
             echo "Configuring the Postgres database ..."
             psql -q -h "${SYMFONY__ENV__DATABASE_HOST}" -p "${SYMFONY__ENV__DATABASE_PORT}" -U "${POSTGRES_USER}" \
                 -c "CREATE DATABASE ${SYMFONY__ENV__DATABASE_NAME};"
+        fi
+
+        TABLES_EXIST="$(psql -qAt -h "${SYMFONY__ENV__DATABASE_HOST}" -p "${SYMFONY__ENV__DATABASE_PORT}" -U "${POSTGRES_USER}" \
+            -c "SELECT 1 FROM pg_catalog.pg_tables WHERE schemaname = 'public';")"
+        if [ "$TABLES_EXIST" == "" ]; then
+            echo "Installing Wallabag ..."
             install_wallabag
         else
             echo "WARN: Postgres database is already configured. Remove the environment variable with root password."