allow overriding workdir for container jobs

Describe the enhancement
Allow overriding container's workdir.
Currently, even if passing --workdir=/somedir to the container: options: it does not override the first --workdir option passed to docker create by the runner.
It would be nice to be able to override the workdir somehow, so that on self-hosted runners, after the job is done the checked out sources would not remain the in actions-runner/_work directory. In container jobs it makes no sense to mount the external directory, as all the job steps are executed within the container anyway. If work dir is set to some in-container directory, then after the job is done the container is discarded and all the build artifacts are discarded as well, no separate cleanup is needed.

Code Snippet

container:
  image: debian:buster
  options: --workdir=/home/build/myrepo

This example currently will result in that --workdir is passed twice to the docker create. The first one is the default one which is --workdir /__w/myrepo/myrepo and the second is the one indicated in options:, but it looks like docker create takes the first one and ignores the second one. So, there is no possibility to override the workdir.
Need to exclude the default --workdir option in case options: contains custom --workdir option.

Start at the runner's Docker container-job handling and the docker create command construction, especially parsing container: options:. Verify how repeated --workdir arguments are handled. Done means a custom --workdir in options: prevents the default workdir from being passed and the container uses the requested path.