fix(deployment): properly escape shell arguments in nixpacks commands

Add escapeShellValue() helper function to safely escape shell values by wrapping
them in single quotes and escaping embedded quotes. Use this function throughout
the nixpacks command building to prevent shell injection vulnerabilities when
passing user-provided build commands, start commands, and environment variables.

This fixes unsafe string concatenation that could allow command injection when
user input contains special shell characters like &&, |, ;, etc.

bootstrap/helpers/docker.php

@@ -137,6 +137,11 @@ function checkMinimumDockerEngineVersion($dockerVersion)
 
     return $dockerVersion;
 }
+function escapeShellValue(string $value): string
+{
+    return "'".str_replace("'", "'\\''", $value)."'";
+}
+
 function executeInDocker(string $containerId, string $command)
 {
     $escapedCommand = str_replace("'", "'\\''", $command);