feat(providers): add base image support and improve forward compatibility

Add support for configuring base images across all compute providers:
- E2B: Accept optional `template` parameter to select custom templates
- Modal: Accept optional `image` parameter (string or Image object) for base images
- ComputeSDK: Expand `create` override to accept full CreateSandboxOptions payload (image, templateId, etc.)
- Daytona: Improve type safety for `image` option

Improve forward compatibility by making all `create` overrides accept full Partial SDK types, allowing any new provider fields to flow through without code changes. Fix Modal provider bug where `encryptedPorts` was hardcoded and would clobber user-provided values; now merges additional ports instead.

Update docs and examples to demonstrate base image configuration for E2B, Modal, and ComputeSDK. Add comprehensive provider lifecycle tests for Modal and ComputeSDK, including template and image passthrough verification.

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

docs/deploy/computesdk.mdx

@@ -27,7 +27,11 @@ if (process.env.OPENAI_API_KEY) envs.OPENAI_API_KEY = process.env.OPENAI_API_KEY
 
 const sdk = await SandboxAgent.start({
   sandbox: computesdk({
-    create: { envs },
+    create: {
+      envs,
+      image: process.env.COMPUTESDK_IMAGE,
+      templateId: process.env.COMPUTESDK_TEMPLATE_ID,
+    },
   }),
 });
 
@@ -43,6 +47,7 @@ try {
 ```
 
 The `computesdk` provider handles sandbox creation, Sandbox Agent installation, agent setup, and server startup automatically. ComputeSDK routes to your configured provider behind the scenes.
+The `create` option now forwards the full ComputeSDK sandbox-create payload, including provider-specific fields such as `image` and `templateId` when the selected provider supports them.
 
 Before calling `SandboxAgent.start()`, configure ComputeSDK with your provider:
 

docs/deploy/e2b.mdx

@@ -21,9 +21,11 @@ import { e2b } from "sandbox-agent/e2b";
 const envs: Record<string, string> = {};
 if (process.env.ANTHROPIC_API_KEY) envs.ANTHROPIC_API_KEY = process.env.ANTHROPIC_API_KEY;
 if (process.env.OPENAI_API_KEY) envs.OPENAI_API_KEY = process.env.OPENAI_API_KEY;
+const template = process.env.E2B_TEMPLATE;
 
 const sdk = await SandboxAgent.start({
   sandbox: e2b({
+    template,
     create: { envs },
   }),
 });
@@ -41,7 +43,10 @@ try {
 
 The `e2b` provider handles sandbox creation, Sandbox Agent installation, agent setup, and server startup automatically. Sandboxes pause by default instead of being deleted, and reconnecting with the same `sandboxId` resumes them automatically.
 
+Pass `template` when you want to start from a custom E2B template alias or template ID. E2B base-image selection happens when you build the template, then `sandbox-agent/e2b` uses that template at sandbox creation time.
+
 ## Faster cold starts
 
 For faster startup, create a custom E2B template with Sandbox Agent and target agents pre-installed.
-See [E2B Custom Templates](https://e2b.dev/docs/sandbox-template).
+Build System 2.0 also lets you choose the template's base image in code.
+See [E2B Custom Templates](https://e2b.dev/docs/sandbox-template) and [E2B Base Images](https://e2b.dev/docs/template/base-image).

docs/deploy/modal.mdx

@@ -21,9 +21,11 @@ import { modal } from "sandbox-agent/modal";
 const secrets: Record<string, string> = {};
 if (process.env.ANTHROPIC_API_KEY) secrets.ANTHROPIC_API_KEY = process.env.ANTHROPIC_API_KEY;
 if (process.env.OPENAI_API_KEY) secrets.OPENAI_API_KEY = process.env.OPENAI_API_KEY;
+const baseImage = process.env.MODAL_BASE_IMAGE ?? "node:22-slim";
 
 const sdk = await SandboxAgent.start({
   sandbox: modal({
+    image: baseImage,
     create: { secrets },
   }),
 });
@@ -40,6 +42,7 @@ try {
 ```
 
 The `modal` provider handles app creation, image building, sandbox provisioning, agent installation, server startup, and tunnel networking automatically.
+Set `image` to change the base Docker image before Sandbox Agent and its agent binaries are layered on top. You can also pass a prebuilt Modal `Image` object.
 
 ## Faster cold starts