Make Selenium image configurable

In certain environments, we may want to customise the docker image e.g.
to use a proxy to avoid docker hub rate limiting. The default behaviour
remains unchanged.

Author: Zerpet

selenium/bin/components/selenium

@@ -1,10 +1,12 @@
 #!/usr/bin/env bash
 
-arch=$(uname -a)
-if [[ $arch == arm* ]]; then
-  SELENIUM_DOCKER_IMAGE=selenium/standalone-chrome:123.0
-else
-  SELENIUM_DOCKER_IMAGE=seleniarm/standalone-chromium:123.0
+if [[ -z "$SELENIUM_DOCKER_IMAGE" ]]; then
+  arch=$(uname -a)
+  if [[ $arch == arm* ]]; then
+    SELENIUM_DOCKER_IMAGE=selenium/standalone-chrome:123.0
+  else
+    SELENIUM_DOCKER_IMAGE=seleniarm/standalone-chromium:123.0
+  fi
 fi
 
 start_selenium() {