Restructure Dockerfile and remove packer for docker builds

[biancabnd]

This PR does the following:

• Restructures the Dockerfile used to build tailor-image (parent) to optimize the number of layers and final image size.
• Removes packer from being used when the image that needs to be built is of type docker. Use native docker build to obtain a multi-stage image

Jenkins pipeline that implements these changes is: http://tailor.locusbots.io/blue/organizations/jenkins/ci%2Frosdistro/detail/feature%2Fpipeline-improvements/3/pipeline