fix(railpack): fail fast when buildx is unavailable

Require Docker buildx before Railpack builds, normalize environment
variable keys before validation, and align private deploy key API docs with
the supported dockerfile build pack.

tests/Unit/ValidationPatternsTest.php

@@ -1,5 +1,6 @@
 <?php
 
+use App\Models\EnvironmentVariable;
 use App\Support\ValidationPatterns;
 
 it('accepts valid names with common characters', function (string $name) {
@@ -165,3 +166,10 @@ it('generates environment variable key rules with correct defaults', function ()
 it('normalizes environment variable keys by trimming surrounding whitespace', function () {
     expect(ValidationPatterns::normalizeEnvironmentVariableKey(' node.name '))->toBe('node.name');
 });
+
+it('normalizes environment variable keys before model validation', function () {
+    $environmentVariable = new EnvironmentVariable;
+    $environmentVariable->key = ' APP_ENV ';
+
+    expect($environmentVariable->key)->toBe('APP_ENV');
+});