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.

The `if` logic was broken because `uname -a` returns the entire uname,
including OS, Kernel version, machine type and what not. The string
always starts with the OS i.e. Linux or Darwin, therefore, the matching
for `arm*` was always false; therefore, it was always defaulting to the
`else` image, which happens to be multi-arch. However, it was using
`seleniarm`, which is a community driven effort, not the official
Selenium account.

In the official OSS image, version 123.0 is too old. The oldest
available is 127.0. This commit bumps to the latest available. We could
consider depending on version `4`. Version `4` refers to Selenium
version, whilst version 123.0/133.0 refer to the browser version.

(cherry picked from commit ef8b4fc767)

selenium/bin/components/selenium

@@ -1,11 +1,8 @@
 #!/usr/bin/env bash
 
-arch=$(uname -a)
-if [[ $arch == arm* ]]; then
-  SELENIUM_DOCKER_IMAGE=selenium/standalone-chrome:123.0
-else
-  SELENIUM_DOCKER_IMAGE=seleniarm/standalone-chromium:123.0
-fi
+# selenium/standalone-chromium is multi-arch
+# https://hub.docker.com/r/selenium/standalone-chromium/tags
+SELENIUM_DOCKER_IMAGE=${SELENIUM_DOCKER_IMAGE:-selenium/standalone-chromium:133.0}
 
 start_selenium() {
   begin "Starting selenium  ..."