Refactor Docker build and add Performer Docker Build validation to gerrit trigger

Motivation

Add Docker Build validation job to the Gerrit trigger to check if new changes don't break tagging in older versions.
The Dockerfile for .NET will have to be modified and improved to allow this change. The Dockerfile should pull couchbase-net-client instead of relying on an existing submodule clone to build.
The build steps in jenkins-sdk will also have to be modified to use the new clone/directory, similar to how the C++ performer is built.
Support for building from gerrit should also be added to jenkins-sdk.

The Gerrit trigger lives here: https://sdk.jenkins.couchbase.com/job/dotnet/job/sdk/job/transactions-dotnet-performer-cv-trigger/

The Docker Performer Validation job lives here: https://sdk.jenkins.couchbase.com/job/fit/job/fit-validate-performers-build/