Fix node version to 16

Temporary workaround for:

Error message "error:0308010C:digital envelope routines::unsupported"

See e.g. discussion at https://stackoverflow.com/questions/69692842/error-message-error0308010cdigital-envelope-routinesunsupported

This is intended to solve the immediate problem of a broken build; when we are back up and running, I'll ticket an upgrade to more recent node.

.gitlab-ci.yml

@@ -4,7 +4,7 @@ stages:
 
 build-esap-gui:
     stage: build
-    image: node:lts
+    image: node:16
     before_script:
       - 'command -v ssh-agent >/dev/null || ( apt-get update -y && apt-get install openssh-client -y )'
     script: