OHK , I completely mis predicted the question and my solution is completely unintended and thats the reason i am sharing it.
So we were given a Docker file and a XML file which we have to tweak and pass into the service to get flag.So if we pass XML then it must be parsing XML and i tried XML injection(XXE) which eventually worked. So this my XML file 
`solution.xml````  ]> <change-log> <text>&systemEntity;</text> </change-log>```
And it worked like a rockstar.
Damn I am soo noob to solve it the intended way :( 