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docs: distinguish the ${WORKDIR} placeholder from the WORKSPACE_DIR variable #26230

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docs: distinguish the ${WORKDIR} placeholder from the WORKSPACE_DIR v…
mdelapenya Sep 30, 2026
42c38b4
docs: lead with what kit authors write, then when it is substituted
mdelapenya Oct 2, 2026
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12 changes: 12 additions & 0 deletions content/manuals/ai/sandboxes/customize/author/_index.md
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Original file line number Diff line number Diff line change
Expand Up @@ -176,6 +176,18 @@ for setup examples and the upstream
[lifecycle definition](https://github.com/docker/sandbox-kit-spec/blob/main/docs/spec/capabilities/com.docker.sandbox/lifecycle@1.md)
for the fields.

### Generated files

In lifecycle `files` content, write `${{ kit.env.NAME }}` to insert a
container environment variable. For example, write
`${{ kit.env.WORKSPACE_DIR }}` for the workspace path. Plain `$VAR` and
`${VAR}` are written unchanged, without substituting their values. In hook
commands, use shell syntax such as `$WORKSPACE_DIR` instead.

Docker Sandboxes substitutes `${{ kit.env.NAME }}` once, when it creates the
sandbox. `kit.env` reads the final container environment, independently of a
hook's `env` list.

## Set workload compute requirements

Set the workload's default CPU and memory allocation with the
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22 changes: 20 additions & 2 deletions content/manuals/ai/sandboxes/customize/kits-v2.md
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Expand Up @@ -488,7 +488,9 @@ assets from `files/home/`.

### startup

Runs at every sandbox start. String array, not interpreted by a shell.
Runs at every sandbox start. String array, not interpreted by a shell. A
`$WORKSPACE_DIR` reference in the array is passed unchanged. To expand it, run
the command through a shell, for example `["sh", "-c", "..."]`.

| Field | Default | Description |
| ------------- | -------- | ----------------------------------- |
Expand Down Expand Up @@ -521,7 +523,7 @@ Files written at sandbox start, with runtime substitution.
| Field | Default | Description |
| --------------- | -------- | --------------------------------------------------------- |
| `path` | — | Absolute container path. |
| `content` | — | File content. `${WORKDIR}` expands to the workspace path. |
| `content` | — | File content. Write `${WORKDIR}` for the workspace path. |
| `mode` | `"0644"` | File permissions in octal. |
| `onlyIfMissing` | `false` | Skip if the file already exists. |

Expand All @@ -530,6 +532,21 @@ path must be writable by that user. To write to a root-owned path such as
`/etc`, use an `install` command, which runs as root by default. Set ownership
in the install command if the agent needs to modify the file later.

Write `${WORKDIR}` in `content` and `$WORKSPACE_DIR` in commands. Three names
look alike and mean different things:

| Name | What it is | Where it applies |
| --------------- | ------------------------------------------------------------- | ------------------------------------------------ |
| `${WORKDIR}` | Placeholder for the workspace path | `setup.files[].content` only |
| `WORKSPACE_DIR` | Environment variable set by the runtime to the workspace path | Shell commands, and the agent |
| `WORKDIR` | The template image's Dockerfile working directory | Where commands start, see [install](#install) |

The runtime replaces `${WORKDIR}` when it creates the sandbox, only inside
`content`, never in `path` or in commands. `WORKSPACE_DIR` is an environment
variable, so a `$WORKSPACE_DIR` reference expands only when a shell runs the
command. `install` strings run through `sh -c`. `startup` arrays run without a
shell. A Dockerfile `WORKDIR` doesn't define an environment variable.

### Shell initialization and service logs

With Docker templates, append shell initialization to
Expand Down Expand Up @@ -792,6 +809,7 @@ content from sandbox initialization, and declare runtime capabilities:
| Automatic `files/home/` and `files/workspace/` injection | Dockerfile `COPY`, with lifecycle hooks for destinations provided by runtime mounts |
| `permissions.network` and `credentials` | Network-policy and credential capabilities |
| `agentInstructions` | Agent-context capability |
| `${WORKDIR}` in `setup.files[].content` | `${{ kit.env.WORKSPACE_DIR }}` in lifecycle [`files` content](/manuals/ai/sandboxes/customize/author/_index.md#generated-files) |

Follow the [v3 authoring guidance](/manuals/ai/sandboxes/customize/author/_index.md)
when converting runtime setup and capability declarations.
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