When `checkout` or `git` steps run inside the Kubernetes plugin's `container` block the Git CLI is expected to be executed on the specified container but it is not. It is executed in the jnlp container disregarding the `container` step.
Below is a test Pipeline. It relies on the custom docker image (duemir/jenkins-jnlp-agent:alpine-no-git) which is based on the `jenkins/jnlp-slave:alpine` Dockerfile but with `git` installation removed. The Console Output is attached.
Git plugin 3.9.1
Git Client plugin 2.7.3
Kuberentes plugin 1.12.8