Overview for published Docker image contains non-applicable information

[javaducky]

The overview provided for the image in Dockerhub includes sections from README that do not apply to the image.

Should remove the "Local Installation" and "Library Usage" sections. The "Command usage" section should be modified as it describes the usage of the binary itself, which is not exactly how the image would be used.

[imiric]

Yeah, I suppose we should have custom documentation just for the Docker image.

Agree with removing the "Local Installation" and "Library Usage" sections. But the "Command Usage" section is still relevant, since the image entrypoint is xk6. Though we should change the examples that reference paths on the local filesystem, since those would have to be mounted in the container first.

@javaducky Do you have bandwidth to do these changes? Maybe propose a PR to this repo as a new document (README-docker.md?), and then we'll update the Docker Hub overview with it.