Docker container is a lightweight execution environment where a user can run their applications.
These are working application created from docker images and a user can start an instance of the application. All the necessary resource and packages to run an application is availble in the docker image.
A container created will have all the root file system that of a linux environment f the image is of linux architecure.
for eg: Below is the container started from an httpd docker image.
Docker container root folder structure
The root folder structure of a docker container is similar to the rootfs of a linux environemnt.
A developer if need to run a java application, They can create docker image with java packages. Once this image is pulled the user can use java commands inside the container start their applications