Hey @akelad thanks for the support! And @degiz thanks a ton for helping over the call and solving issue!! 
I am documenting the solutions you provided for people who might stumble across the same issues.
For Linux server:
Root issue was that actions.actions was not getting registered.
- My installation folder was different than /etc/rasa and somehow probably I missed the step to change RASA_HOME variable to the directory I had installation in. (details given in step 2 here)
- The actions.py in my project had dependencies which are not present in default rasa-sdk image. So, created Dockerfile as mentioned here and created custom_sdk
And finally actions.actions got registered!! Yay!!
Then there was ‘File not found error’ for which I got a hint from this post. The problem was, my actions.py was using local files which needed to be included while forming image.
After successfully copying files in the image, then I faced issue with action server not running! So, I explored a bit more to find that my ‘endpoints.yml’ had an issue, it was pointing to localhost i.e. ‘http://localhost:5055/webhook’. The correct URL should be ‘http://<image_name_as_in_docker-compose.yml>:5055/webhook’ which, in my case was ‘app’.
And finallieee Rasa X is up and running flawlessly!! 