Docker, is a new container technology.
Docker is being adopted by companies at enormous rate, particularly the BFSI IT companies.
Docker is open source technology. Concept of containers started around year 2000.
It makes it possible to get far more apps running on the same old servers and it also makes it very easy to package and ship programs.
If you want your server to run most application instances executing on least amount of hardware, Docker is the fit case to be used there.
However if the need is to run multiple applications on different operating systems on same hardware, Docker is not the tool to be used.
Leaving these apart Docker pack/ ship and run any application that can virtually run anywhere.