Fix builder resource flags being reset during builds

Sources/ContainerCommands/Builder/BuilderStart.swift

@@ -126,7 +126,7 @@ extension Application {
 
             let defaultBuildCPUs: Int = containerSystemConfig.build.cpus
             let defaultBuildMemory = containerSystemConfig.build.memory
-            let resources = try Parser.resources(
+            var resources = try Parser.resources(
                 cpus: cpus,
                 memory: memory,
                 defaultCPUs: defaultBuildCPUs,
@@ -138,6 +138,12 @@ extension Application {
             if let existingContainer {
                 let existingImage = existingContainer.configuration.image.reference
                 let existingResources = existingContainer.configuration.resources
+                resources = BuilderStart.resourcesForStart(
+                    requested: resources,
+                    existing: existingResources,
+                    cpus: cpus,
+                    memory: memory
+                )
                 let existingEnv = existingContainer.configuration.initProcess.environment
                 let existingDNS = existingContainer.configuration.dns
 
@@ -323,6 +329,23 @@ extension Application {
             try await startBuildKit(client: client, id: Builder.builderContainerId, progressUpdate, taskManager)
             log.debug("starting BuildKit and BuildKit-shim")
         }
+
+        // A nil flag means the caller expressed no preference, so preserve the existing value.
+        static func resourcesForStart(
+            requested: ContainerConfiguration.Resources,
+            existing: ContainerConfiguration.Resources,
+            cpus: Int64?,
+            memory: String?
+        ) -> ContainerConfiguration.Resources {
+            var resources = requested
+            if cpus == nil {
+                resources.cpus = existing.cpus
+            }
+            if memory == nil {
+                resources.memoryInBytes = existing.memoryInBytes
+            }
+            return resources
+        }
     }
 }
 

Tests/ContainerCommandsTests/BuilderStartTests.swift

@@ -0,0 +1,78 @@
+//===----------------------------------------------------------------------===//
+// Copyright © 2026 Apple Inc. and the container project authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//   https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//===----------------------------------------------------------------------===//
+
+import ContainerResource
+import Foundation
+import Testing
+
+@testable import ContainerCommands
+
+struct BuilderStartTests {
+    private func resources(cpus: Int, mebibytes: UInt64) -> ContainerConfiguration.Resources {
+        var resources = ContainerConfiguration.Resources()
+        resources.cpus = cpus
+        resources.memoryInBytes = mebibytes * 1024 * 1024
+        return resources
+    }
+
+    @Test
+    func startingWithoutResourceFlagsPreservesExistingResources() {
+        let requested = resources(cpus: 2, mebibytes: 2048)
+        let existing = resources(cpus: 6, mebibytes: 12288)
+
+        let result = Application.BuilderStart.resourcesForStart(
+            requested: requested,
+            existing: existing,
+            cpus: nil,
+            memory: nil
+        )
+
+        #expect(result.cpus == 6)
+        #expect(result.memoryInBytes == 12288 * 1024 * 1024)
+    }
+
+    @Test
+    func startingWithCpuFlagPreservesExistingMemory() {
+        let requested = resources(cpus: 2, mebibytes: 2048)
+        let existing = resources(cpus: 6, mebibytes: 12288)
+
+        let result = Application.BuilderStart.resourcesForStart(
+            requested: requested,
+            existing: existing,
+            cpus: 2,
+            memory: nil
+        )
+
+        #expect(result.cpus == 2)
+        #expect(result.memoryInBytes == 12288 * 1024 * 1024)
+    }
+
+    @Test
+    func startingWithMemoryFlagPreservesExistingCpus() {
+        let requested = resources(cpus: 2, mebibytes: 4096)
+        let existing = resources(cpus: 6, mebibytes: 12288)
+
+        let result = Application.BuilderStart.resourcesForStart(
+            requested: requested,
+            existing: existing,
+            cpus: nil,
+            memory: "4G"
+        )
+
+        #expect(result.cpus == 6)
+        #expect(result.memoryInBytes == 4096 * 1024 * 1024)
+    }
+}