Dockerize example application

This MR is to build a docker image for example-app. It should build an image for every commit to main branch and it will be tagged with branch name, main in this case. MR needs to get merged to main branch for the job to get triggered.

Few extra things apart from building the container in CI had to be updated:

• Added environment variables PA_COLLECTOR_HOST, PA_COLLECTOR_PORT, PA_APPLICATION_ID to example app in order to control Snowplow collector host, Snowplow collector port and Product Analytics application ID.
• Updated applicationURL to http://0.0.0.0:5171 as it can not be reached from outside of docker container in case aplicationURL set to http://localhost:5171