Use fixed version selenium-chrome image

package.json

@@ -9,7 +9,7 @@
  "scripts": {
  "mermaid": "for i in ./docs/architecture/*.mmd; do mmdc -i $i -o $i.png -t neutral; done",
  "test": "mocha --reporter mocha-junit-reporter --recursive tests",
- "art:pre": "(docker kill bigboat-art-selenium-server; sleep 3; docker run --rm -d --net=host --name=bigboat-art-selenium-server -e SCREEN_WIDTH=1920 -e SCREEN_HEIGHT=1080 selenium/standalone-chrome)",
+ "art:pre": "(docker kill bigboat-art-selenium-server; sleep 3; docker run --rm -d --net=host --name=bigboat-art-selenium-server -e SCREEN_WIDTH=1920 -e SCREEN_HEIGHT=1080 selenium/standalone-chrome:3.12)",
  "art:exec": "protractor tests/testx/conf.coffee --seleniumAddress=http://localhost:4444/wd/hub",
  "art:post": "docker kill bigboat-art-selenium-server",
  "art": "run-s art:pre \"art:exec -- {@}\" art:post --",