fix(railpack): interpolate build-time env variables by sourcing build-time .env

Railpack builds forwarded build-time variables inline as `env 'KEY=VALUE'`,
which single-quotes each value and prevents shell interpolation. References
like BETTER_AUTH_URL=$COOLIFY_URL reached the build as the literal string
"$COOLIFY_URL" instead of the resolved URL, breaking builds that validate
their env (e.g. SvelteKit/better-auth).

Wrap the railpack `docker buildx build` invocation with the same
wrap_build_command_with_env_export() helper used by the Dockerfile and
Nixpacks build paths, sourcing the build-time .env file (which writes
COOLIFY_* first and double-quotes normal vars to allow $VAR expansion).
Only buildpack control variables (NIXPACKS_/RAILPACK_), excluded from that
file and never needing interpolation, remain inline. Secret flags are
unchanged and read the interpolated values from the exported environment.

Fixes #10736

app/Jobs/ApplicationDeploymentJob.php

@@ -2667,12 +2667,22 @@ private function railpack_build_command(string $imageName, Collection $variables
             $cacheArgs .= ' --build-arg secrets-hash='.$this->generate_secrets_hash($variables);
         }
 
-        $environmentPrefix = $this->railpack_build_environment_prefix($variables);
+        // Build-time variables reach the build through the sourced build-time .env file
+        // (written by save_buildtime_environment_variables), which interpolates shell-style
+        // references such as BETTER_AUTH_URL=$COOLIFY_URL. Passing them inline via `env`
+        // would forward the literal `$COOLIFY_URL` because each value is single-quoted and
+        // `env` does not interpolate its own assignments. Only buildpack control variables
+        // (NIXPACKS_/RAILPACK_) — which are excluded from the build-time .env file and never
+        // need interpolation — are still passed inline.
+        $controlVariables = $variables->filter(
+            fn ($value, $key) => str($key)->startsWith(EnvironmentVariable::BUILDPACK_CONTROL_VARIABLE_PREFIXES)
+        );
+
+        $environmentPrefix = $this->railpack_build_environment_prefix($controlVariables);
         $secretFlags = $this->railpack_build_secret_flags($variables);
         $frontendImage = 'ghcr.io/railwayapp/railpack-frontend:v'.config('constants.coolify.railpack_version');
 
-        return 'docker buildx create --name coolify-railpack --driver docker-container 2>/dev/null || true'
+        $buildxBuildCommand = "{$environmentPrefix}docker buildx build --builder coolify-railpack"
-            ." && {$environmentPrefix}docker buildx build --builder coolify-railpack"
             ." {$this->addHosts} --network host"
             ." --build-arg BUILDKIT_SYNTAX=\"{$frontendImage}\""
             ." {$cacheArgs}"
@@ -2682,6 +2692,9 @@ private function railpack_build_command(string $imageName, Collection $variables
             .' --load'
             ." -t {$imageName}"
             ." {$this->workdir}";
+
+        return 'docker buildx create --name coolify-railpack --driver docker-container 2>/dev/null || true'
+            .' && '.$this->wrap_build_command_with_env_export($buildxBuildCommand);
     }
 
     private function decode_railpack_config(string $config, string $source): array

tests/Unit/ApplicationDeploymentRailpackConfigTest.php

@@ -236,10 +236,15 @@ function invokeRailpackMethod(object $job, ReflectionClass $reflection, string $
         ],
     );
 
+    // Build-time variables are interpolated by sourcing the build-time .env file before
+    // the build, so user/Coolify variables must NOT be forwarded inline as literals.
+    expect($command)->toContain('set -a && source /artifacts/build-time.env && set +a');
     expect($command)->toContain("env 'RAILPACK_NODE_VERSION=22'");
     expect($command)->toContain("'RAILPACK_INSTALL_CMD=npm ci && npm run postinstall'");
     expect($command)->toContain("'RAILPACK_DEPLOY_APT_PACKAGES=curl wget'");
-    expect($command)->toContain("'SECRET_JSON={\"token\":\"abc\"}'");
+    // SECRET_JSON is not a buildpack control variable, so it is provided via the sourced
+    // build-time .env file (which supports $VAR interpolation) rather than inline `env`.
+    expect($command)->not->toContain("'SECRET_JSON={\"token\":\"abc\"}'");
     expect($command)->toContain("--secret 'id=RAILPACK_NODE_VERSION,env=RAILPACK_NODE_VERSION'");
     expect($command)->toContain("--secret 'id=RAILPACK_INSTALL_CMD,env=RAILPACK_INSTALL_CMD'");
     expect($command)->toContain("--secret 'id=RAILPACK_DEPLOY_APT_PACKAGES,env=RAILPACK_DEPLOY_APT_PACKAGES'");
@@ -247,3 +252,31 @@ function invokeRailpackMethod(object $job, ReflectionClass $reflection, string $
     expect($command)->toContain(' --build-arg secrets-hash=');
     expect($command)->toContain('--build-arg BUILDKIT_SYNTAX="ghcr.io/railwayapp/railpack-frontend:v'.config('constants.coolify.railpack_version').'"');
 });
+
+it('interpolates build-time variable references for railpack by sourcing the build-time env file', function () {
+    [$job, $reflection] = makeRailpackDeploymentJob([
+        'uuid' => 'application-uuid',
+    ]);
+
+    // Mirrors the issue: BETTER_AUTH_URL=$COOLIFY_URL must be interpolated at build time.
+    $command = invokeRailpackMethod(
+        $job,
+        $reflection,
+        'railpack_build_command',
+        [
+            'coollabsio/coolify:test',
+            collect([
+                'BETTER_AUTH_URL' => '$COOLIFY_URL',
+                'COOLIFY_URL' => 'https://sapere-10.bobman.dev',
+            ]),
+        ],
+    );
+
+    // The literal `$COOLIFY_URL` must NOT be forwarded inline; it is resolved by the shell
+    // after sourcing the build-time .env file, then read through the build secret.
+    expect($command)->toContain('set -a && source /artifacts/build-time.env && set +a');
+    expect($command)->not->toContain("'BETTER_AUTH_URL=\$COOLIFY_URL'");
+    expect($command)->not->toContain("env 'BETTER_AUTH_URL");
+    expect($command)->toContain("--secret 'id=BETTER_AUTH_URL,env=BETTER_AUTH_URL'");
+    expect($command)->toContain("--secret 'id=COOLIFY_URL,env=COOLIFY_URL'");
+});