docs: document sandbox provider selection via RUNTIME

Co-authored-by: openhands <openhands@all-hands.dev>

Author: enyst

openhands/usage/advanced/configuration-options.mdx

@@ -42,6 +42,18 @@ These are some commonly used variables in V1 deployments:
   - <code>AGENT_SERVER_IMAGE_REPOSITORY</code>
   - <code>AGENT_SERVER_IMAGE_TAG</code>
 
+
+## Sandbox provider selection
+
+Some deployments still use the legacy <code>RUNTIME</code> environment variable to
+choose which sandbox provider to use:
+
+- <code>RUNTIME=docker</code> (default)
+- <code>RUNTIME=process</code> (aka legacy <code>RUNTIME=local</code>)
+- <code>RUNTIME=remote</code>
+
+See [Sandboxes overview](/openhands/usage/sandboxes/overview) for details.
+
 ## Need legacy options?
 
 If you are looking for the old <code>config.toml</code> reference or V0 “runtime”

openhands/usage/sandboxes/docker.mdx

@@ -6,6 +6,12 @@ description: The recommended sandbox provider for running OpenHands locally.
 The **Docker sandbox** runs the agent server inside a Docker container. This is
 the default and recommended option for most users.
 
+<Note>
+  In some self-hosted deployments, the sandbox provider is controlled via the
+  legacy <code>RUNTIME</code> environment variable. Docker is the default.
+</Note>
+
+
 ## Why Docker?
 
 - Isolation: reduces risk when the agent runs commands.

openhands/usage/sandboxes/overview.mdx

@@ -24,6 +24,20 @@ OpenHands supports multiple sandbox “providers”, with different tradeoffs:
   - Runs the agent server in a remote environment.
   - Used by managed deployments and some hosted setups.
 
+## Selecting a provider (current behavior)
+
+In some deployments, the provider selection is still controlled via the legacy
+<code>RUNTIME</code> environment variable:
+
+- <code>RUNTIME=docker</code> (default)
+- <code>RUNTIME=process</code> (aka legacy <code>RUNTIME=local</code>)
+- <code>RUNTIME=remote</code>
+
+<Note>
+  The user-facing terminology in V1 is <b>sandbox</b>, but the configuration knob
+  may still be called <code>RUNTIME</code> while the migration is in progress.
+</Note>
+
 ## Terminology note (V0 vs V1)
 
 Older documentation refers to these environments as **runtimes**.

openhands/usage/sandboxes/process.mdx

@@ -23,8 +23,13 @@ regular process.
 
 ## Choosing process mode
 
-Exact configuration depends on how you run OpenHands (CLI launcher, Docker, or
-a self-hosted deployment). In some setups this may still be exposed under a
-legacy “runtime” selector.
+In some deployments, this is selected via the legacy <code>RUNTIME</code>
+environment variable:
+
+```bash
+export RUNTIME=process
+# (legacy alias)
+# export RUNTIME=local
+```
 
 If you are unsure, prefer the [Docker Sandbox](/openhands/usage/sandboxes/docker).

openhands/usage/sandboxes/remote.mdx

@@ -9,6 +9,21 @@ instead of on your local machine.
 This is typically used by managed deployments (e.g., OpenHands Cloud) and
 advanced self-hosted setups.
 
+## Selecting remote mode
+
+In some self-hosted deployments, remote sandboxes are selected via the legacy
+<code>RUNTIME</code> environment variable:
+
+```bash
+export RUNTIME=remote
+```
+
+Remote sandboxes require additional configuration (API URL + API key). The exact
+variable names depend on your deployment, but you may see legacy names like:
+
+- <code>SANDBOX_REMOTE_RUNTIME_API_URL</code>
+- <code>SANDBOX_API_KEY</code>
+
 ## Notes
 
 - Remote sandboxes may expose additional service URLs (e.g., VS Code, app ports)