Prevent Dockerfile from being modified during Converter specs

Why is this change being made?

The Converter spec currently has a side effect of editing the project's Dockerfile when it is run. This is because Gcs::Environment#docker_file picks up the Dockerfile in the current working directory as the default value. This MR fixes the behavior by having Gcs::Environment#docker_file return a file which does not exist during the Converter specs, causing remediation to be skipped. This does not reduce test coverage since remediations are tested individually by their own specs.