fix(ci-templates): allow to set docker build context in Docker.gitlab-ci.yml

What does this MR do and why?

Not possible to set the container build context in the gitlab ci file template : https://gitlab.com/gitlab-org/gitlab/-/blob/master/lib/gitlab/ci/templates/Docker.gitlab-ci.yml
But in the gitlab ci file template is possible : https://gitlab.com/gitlab-org/gitlab/-/blob/master/lib/gitlab/ci/templates/Kaniko.gitlab-ci.yml
I just add the possibily to set it by using a environment variable.

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