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

Dockerfile

@@ -20,6 +20,7 @@ ENV LANG=C.UTF-8 \
     COREPACK_ENABLE_DOWNLOAD_PROMPT=0 \
     COREPACK_ENABLE_AUTO_PIN=0 \
     PYENV_ROOT=/usr/local/pyenv \
+    PLAYWRIGHT_BROWSERS_PATH=/ms-playwright \
     GOPATH=/root/go \
     PATH=/usr/local/go/bin:/root/go/bin:/usr/local/nvm/current/bin:/usr/local/pyenv/shims:/usr/local/pyenv/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin