Create volume for caching playwright browsers

Playwright must download and install browsers before it can run any
tests. Users can do this (inside their devcontainers) by running:

    npx playwright install [chromium [firefox]]

Plenty of other browsers are available.

You don't want to be doing this every time you build a devcontainer, so
we'll cache the downloaded browsers in a named volume. The volume will
still be available after devcontainer images have been torn down and
rebuilt.

The feature configures playwright to automatically use this volume by
setting the PLAYWRIGHT_BROWSERS_PATH environment variable.

src/playwright/devcontainer-feature.json

@@ -4,6 +4,14 @@
     "version": "1.0.0",
     "description": "Install Playwright's dependencies and cache installed browsers",
     "keywords": ["playwright", "testing", "browser-testing"],
+    "containerEnv": {
+        "PLAYWRIGHT_BROWSERS_PATH": "/playwright/browsers"
+    },
+    "mounts": [{
+        "source": "playwright-browsers",
+        "target": "/playwright/browsers",
+        "type": "volume"
+    }],
     "options": {
         "browsers": {
             "type": "string",

test/playwright/browser_volume.sh

@@ -0,0 +1,11 @@
+#!/bin/bash
+
+set -e
+
+source dev-container-features-test-lib
+
+CACHE_DIR="/playwright/browsers"
+
+check "playwright-browsers volume is mounted" test -d "$CACHE_DIR"
+
+check "PLAYWRIGHT_BROWSERS_PATH is set" env | grep "PLAYWRIGHT_BROWSERS_PATH=$CACHE_DIR"

test/playwright/scenarios.json

@@ -1,4 +1,11 @@
 {
+  "browser_volume": {
+    "image": "debian:latest",
+    "features": {
+      "ghcr.io/devcontainers/features/node:1": {},
+      "playwright": {}
+    }
+  },
   "chromium_dependencies": {
     "image": "debian:latest",
     "features": {