ContainerEnv interpretation

- new containerenv plugin, based off of the toolbox plugin
  - interpret the /run/.containerenv file for parametrization
wq  - add a prompt info function that can be used to put container info into the prompt

Signed-off-by: James Nesbitt <james.r.nesbitt@gmail.com>

plugins/containerenv/README.md

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+# containerenv plugin
+
+A lightweight plugin that provides convenient functions to access container environment variables from `/run/.containerenv`. Useful in containerized environments (Podman, Docker, Toolbox, etc.).
+
+To use it, add `containerenv` to the plugins array in your `.zshrc` file:
+
+```zsh
+plugins=(... containerenv)
+```
+
+## Usage
+
+### Accessor functions
+
+Each function reads one key from `/run/.containerenv` and prints its value to stdout. On error (not in a container or missing key), a message is printed to stderr and the function returns 1.
+
+| Function | Key | Description |
+|----------|-----|-------------|
+| `containerenv_engine` | `engine` | Container runtime (e.g. `podman`, `docker`) |
+| `containerenv_name` | `name` | Container name |
+| `containerenv_id` | `id` | Container ID |
+| `containerenv_image` | `image` | Image name/reference |
+| `containerenv_imageid` | `imageid` | Image ID |
+| `containerenv_rootless` | `rootless` | Whether the container is rootless |
+| `containerenv_graphrootmounted` | `graphRootMounted` | Whether the graph root is mounted |
+
+**Examples:**
+
+```bash
+# Show container name
+containerenv_name
+
+# Use in a conditional or variable
+if name=$(containerenv_name 2>/dev/null); then
+  echo "Container: $name"
+fi
+```
+
+### Show all container env vars
+
+```bash
+containerenv_all
+```
+
+Prints the full contents of `/run/.containerenv`. Exits with 1 if not in a container.
+
+### Check if running in a container
+
+```bash
+is_containerized
+```
+
+Returns 0 if `/run/.containerenv` exists, 1 otherwise. Useful in scripts:
+
+```bash
+if is_containerized; then
+  echo "Inside container: $(containerenv_name 2>/dev/null)"
+fi
+```
+
+### Prompt integration
+
+Use `containerenv_prompt_info` in your theme to show container info in the prompt (e.g. `📦 container-name`). If not in a container, it prints nothing. When icons are enabled, the prompt shows an engine-specific icon (🐳 docker, ⬢ podman, 📦 other) and optionally 🔓 for rootless containers.
+
+In a custom theme or in `PROMPT`/`RPROMPT`:
+
+```zsh
+# Example: add to RPROMPT
+RPROMPT='$(containerenv_prompt_info)'
+```
+
+**Environment variable**
+
+| Variable | Description |
+|----------|-------------|
+| `OMZ_CONTAINERENV_PROMPT_NO_ICONS` | Set to any value (e.g. `1`) to disable prompt icons. Only the container name is shown (no engine or rootless icons). When unset, icons are shown. |
+
+---
+
+## About `/run/.containerenv`
+
+Podman (and compatible runtimes) write a file at `/run/.containerenv` inside the container with key=value pairs such as `engine`, `name`, `id`, `image`, `imageid`, `rootless`, and `graphRootMounted`. This plugin reads that file; outside a container or if the file is missing, the accessors report an error.