Add browsers for tests: Playwright, Firefox, geckodriver, headless Chrome (closes #10)

Firefox comes from Mozilla's own apt repo, pinned above Ubuntu's snap-only
package. geckodriver is at its latest release, independent of the Firefox
version. Playwright installs its Chromium and Firefox; WebKit is skipped
because no repository uses it today. Playwright's Chromium is exposed as
the headless Chrome.

Model: deepseek-v4-pro

README.md

@@ -30,6 +30,11 @@ Image: `git.eeqj.de/sneak/baseimage`, built for `linux/amd64` and
 | yarn, pnpm      | corepack, cache pre-populated  | `/usr/local/corepack`          |
 | wrangler        | `npm install -g wrangler`      | nvm LTS global bin             |
 | prettier        | `npm install -g prettier`      | nvm LTS global bin             |
+| Playwright      | `npm install -g playwright`    | nvm LTS global bin             |
+| Playwright browsers | `playwright install` (chromium, firefox) | `/ms-playwright`     |
+| Firefox         | Mozilla apt repo               | `/usr/bin/firefox`             |
+| geckodriver     | GitHub release                 | `/usr/local/bin/geckodriver`   |
+| headless Chrome | Playwright Chromium            | `chromium` on `PATH`           |
 | Python (distro) | apt `python3`, `pip`, `venv`   | `/usr/bin/python3`             |
 | pyenv           | git tag                        | `/usr/local/pyenv`             |
 | Python (latest) | `pyenv install`                | `/usr/local/pyenv/versions/*`  |
@@ -68,7 +73,8 @@ dependencies in child images.
 
 Environment variables set in the image: `NVM_DIR`, `NVM_SYMLINK_CURRENT`,
 `COREPACK_HOME`, `COREPACK_ENABLE_DOWNLOAD_PROMPT=0`,
-`COREPACK_ENABLE_AUTO_PIN=0`, `PYENV_ROOT`, `GOPATH`, `LANG=C.UTF-8`.
+`COREPACK_ENABLE_AUTO_PIN=0`, `PYENV_ROOT`, `GOPATH`, `LANG=C.UTF-8`,
+`PLAYWRIGHT_BROWSERS_PATH`.
 
 ### Yarn and corepack
 
@@ -91,9 +97,18 @@ the `Dockerfile`:
 | `NVM_GIT_TAG`      | `nvm-sh/nvm` release tag   |
 | `PYENV_GIT_TAG`    | `pyenv/pyenv` release tag  |
 
-Everything else resolves to its current release at build time: Ubuntu
-packages, Go, Node LTS, Python, Hugo, golangci-lint, npm, yarn, pnpm,
-wrangler.
+Everything else resolves to its current release at build time:
+
+- Ubuntu packages
+- Go, Node LTS, and Python
+- Hugo and golangci-lint
+- npm, yarn, pnpm, wrangler, and prettier
+- Playwright and its browsers
+- Firefox and geckodriver
+
+Playwright installs its browsers for the image's Playwright version only. A
+child image that pins a different Playwright version downloads its own
+browsers.
 
 ## Services and init