What is the role of 'ENTRYPOINT' in a Dockerfile?

The role of 'ENTRYPOINT' in a Dockerfile is to specify a command that runs at container start. This command is executed when the container is launched, making 'ENTRYPOINT' an essential instruction for determining the primary function of the container.

When you define an 'ENTRYPOINT', you essentially set the default executable that will be run within the container. This allows for greater control over how the container behaves and can enable the inclusion of arguments passed at runtime. For instance, if your container is intended to run a web server, you would set the 'ENTRYPOINT' to the command that starts that server.

Moreover, this instruction can be combined with 'CMD' to supply default arguments to the entry point or to provide a way to override the command when the container is run. By establishing a clear entry point, it ensures that the intended application starts correctly each time the container is executed.

In contrast, the other options represent functionalities that are not related to 'ENTRYPOINT'. Environmental variables are consistent with a different instruction, and defining an image's base OS or setting a default user pertains to other directives within a Dockerfile.