Move HE samples to separate docker layer

[jlhcrawford]

Proposed changes

Separating out the installation of he-samples in Dockerfile.toolkit to its own docker file Dockerfile.samples and making it an optional layer in the docker build.

Types of changes

What types of changes does your code introduce to the HE Toolkit project?
Put an x in the boxes that apply

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update (if none of the other choices apply)

Checklist

Put an x in the boxes that apply. You can also fill these out after creating
the PR. If you are unsure about any of them, do not hesitate to ask. We are
here to help! This is simply a reminder of what we are going to look for before
merging your code.

• I have read the CONTRIBUTING section
• Current formatting and unit tests / base functionality passes locally with my changes
• I have added tests that prove my fix is effective or that my feature works (if appropriate)
• I have added necessary documentation (if appropriate)
• Any dependent changes have been merged and published in downstream modules

Further comments

Dockerfile.toolkit now just sets up the hekit command. Making the samples layer optional allows more flexibility for the user to be able to install/build a custom project or subset of the default.toml recipe file.