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.

app/Models/EnvironmentVariable.php

@@ -371,7 +371,9 @@ class EnvironmentVariable extends BaseModel
     protected function key(): Attribute
     {
         return Attribute::make(
-            set: fn (string $value) => ValidationPatterns::validatedEnvironmentVariableKey($value),
+            set: fn (string $value) => ValidationPatterns::validatedEnvironmentVariableKey(
+                ValidationPatterns::normalizeEnvironmentVariableKey($value)
+            ),
         );
     }