EDIT: As suggested, I've looked in storage/logs/web.log
and this is what I get:
2019-12-05 08:35:08 [-][-][d97bb3d28a3fdd84f8c8edc795e22e6d][info][yii\db\Connection::open] Opening DB connection: mysql:host=localhost;dbname=local;port=4095;
2019-12-05 08:35:08 [-][-][d97bb3d28a3fdd84f8c8edc795e22e6d][profile begin][yii\db\Connection::open] Opening DB connection: mysql:host=localhost;dbname=local;port=4095;
2019-12-05 08:35:08 [-][-][d97bb3d28a3fdd84f8c8edc795e22e6d][profile end][yii\db\Connection::open] Opening DB connection: mysql:host=localhost;dbname=local;port=4095;
2019-12-05 08:35:08 [-][-][d97bb3d28a3fdd84f8c8edc795e22e6d][info][yii\db\Command::query] SHOW TABLES
2019-12-05 08:35:08 [-][-][d97bb3d28a3fdd84f8c8edc795e22e6d][profile begin][yii\db\Command::query] SHOW TABLES
2019-12-05 08:35:08 [-][-][d97bb3d28a3fdd84f8c8edc795e22e6d][profile end][yii\db\Command::query] SHOW TABLES
2019-12-05 08:35:09 [-][-][d97bb3d28a3fdd84f8c8edc795e22e6d][trace][yii\base\Module::getModule] Loading module: exifReturn
2019-12-05 08:35:09 [-][-][d97bb3d28a3fdd84f8c8edc795e22e6d][trace][yii\base\Application::bootstrap] Bootstrap with modules\exifreturn\ExifReturn
2019-12-05 08:35:09 [-][-][d97bb3d28a3fdd84f8c8edc795e22e6d][info][yii\web\Session::open] Session started
2019-12-05 08:35:09 [-][-][d97bb3d28a3fdd84f8c8edc795e22e6d][error][yii\web\HttpException:503] craft\web\ServiceUnavailableHttpException in /app/pb-repo/vendor/craftcms/cms/src/web/Application.php:529
Stack trace:
#0 /app/pb-repo/vendor/craftcms/cms/src/web/Application.php(187): craft\web\Application->_processInstallRequest(Object(craft\web\Request))
#1 /app/pb-repo/vendor/yiisoft/yii2/base/Application.php(386): craft\web\Application->handleRequest(Object(craft\web\Request))
#2 /app/pb-repo/web/index.php(21): yii\base\Application->run()
#3 {main}
2019-12-05 08:35:09 [-][-][d97bb3d28a3fdd84f8c8edc795e22e6d][trace][yii\base\View::renderFile] Rendering view file: /app/pb-repo/vendor/yiisoft/yii2/views/errorHandler/exception.php
2019-12-05 08:35:09 [-][-][d97bb3d28a3fdd84f8c8edc795e22e6d][trace][yii\base\View::renderFile] Rendering view file: /app/pb-repo/vendor/yiisoft/yii2/views/errorHandler/callStackItem.php
2019-12-05 08:35:09 [-][-][d97bb3d28a3fdd84f8c8edc795e22e6d][trace][yii\base\View::renderFile] Rendering view file: /app/pb-repo/vendor/yiisoft/yii2/views/errorHandler/callStackItem.php
2019-12-05 08:35:09 [-][-][d97bb3d28a3fdd84f8c8edc795e22e6d][trace][yii\base\View::renderFile] Rendering view file: /app/pb-repo/vendor/yiisoft/yii2/views/errorHandler/callStackItem.php
2019-12-05 08:35:09 [-][-][d97bb3d28a3fdd84f8c8edc795e22e6d][trace][yii\base\View::renderFile] Rendering view file: /app/pb-repo/vendor/yiisoft/yii2/views/errorHandler/callStackItem.php
2019-12-05 08:35:08 [-][-][d97bb3d28a3fdd84f8c8edc795e22e6d][info][application] $_GET = []
$_POST = []
$_FILES = []
$_COOKIE = [
'CraftSessionId' => 'd97bb3d28a3fdd84f8c8edc795e22e6d'
]
$_SESSION = [
'bd62416aa8538ede709019a5e113eea5__flash' => []
]
$_SERVER = [
'USER' => 'www-data'
'HOME' => '/var/www'
'SCRIPT_NAME' => '/index.php'
'REQUEST_URI' => '/'
'QUERY_STRING' => ''
'REQUEST_METHOD' => 'GET'
'SERVER_PROTOCOL' => 'HTTP/1.0'
'GATEWAY_INTERFACE' => 'CGI/1.1'
'REMOTE_PORT' => '50902'
'SCRIPT_FILENAME' => '//app/pb-repo/web/index.php'
'SERVER_ADMIN' => 'webmaster@localhost'
'CONTEXT_DOCUMENT_ROOT' => '/app/pb-repo/web'
'CONTEXT_PREFIX' => ''
'REQUEST_SCHEME' => 'http'
'DOCUMENT_ROOT' => '/app/pb-repo/web'
'REMOTE_ADDR' => '172.17.0.1'
'SERVER_PORT' => '80'
'SERVER_ADDR' => '172.17.0.5'
'SERVER_NAME' => 'pb.local'
'SERVER_SOFTWARE' => 'Apache/2.4.10 (Debian)'
'SERVER_SIGNATURE' => ''
'PATH' => '/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin'
'HTTP_COOKIE' => 'CraftSessionId=d97bb3d28a3fdd84f8c8edc795e22e6d'
'HTTP_UPGRADE_INSECURE_REQUESTS' => '1'
'HTTP_ACCEPT_ENCODING' => 'gzip, deflate, br'
'HTTP_ACCEPT_LANGUAGE' => 'en-GB,en;q=0.5'
'HTTP_ACCEPT' => 'text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8'
'HTTP_USER_AGENT' => 'Mozilla/5.0 (Macintosh; Intel Mac OS X 10.13; rv:70.0) Gecko/20100101 Firefox/70.0'
'HTTP_CONNECTION' => 'close'
'HTTP_X_FORWARDED_PROTO' => 'https'
'HTTP_X_FORWARDED_HOST' => 'pb.local'
'HTTP_X_FORWARDED_FOR' => '192.168.95.1'
'HTTP_X_REAL_IP' => '192.168.95.1'
'HTTP_HOST' => 'pb.local'
'proxy-nokeepalive' => '1'
'HTTPS' => 'on'
'FCGI_ROLE' => 'RESPONDER'
'PHP_SELF' => '/index.php'
'REQUEST_TIME_FLOAT' => 1575534907.7596
'REQUEST_TIME' => 1575534907
'ALLOW_UPDATES' => '1'
'ALLOW_ADMIN_CHANGES' => '1'
'BACKUP_ON_UPDATE' => '0'
'DEV_MODE' => '1'
'ENABLE_TEMPLATE_CACHING' => '1'
'ENVIRONMENT' => 'dev'
'IS_SYSTEM_LIVE' => '1'
'RUN_QUEUE_AUTOMATICALLY' => '1'
'SECURITY_KEY' => '••••••••••••••••••••••••••••••••••••••••'
'SITE_NAME' => ''
'DB_DRIVER' => 'mysql'
'DB_SERVER' => 'localhost'
'DB_USER' => 'root'
'DB_PASSWORD' => '••••'
'DB_DATABASE' => 'local'
'DB_SCHEMA' => 'public'
'DB_TABLE_PREFIX' => ''
'DB_PORT' => '4095'
'ASSETS_URL' => ''
'UPLOADS_URL' => '/uploads/'
'SITE_URL' => 'https://pb.local/'
'WEB_ROOT_PATH' => '.'
'PUBLIC_PATH' => './web/'
'DEFAULT_SITE_URL' => 'https://pb.local'
]
So it seems to be connecting OK, showing tables, loading some modules, starting the session, and then returning the httpException error.