Restructure container builds to use multi-stage builds and a Merlin base image

[karlhigley]

This also adds container tests with container-structure-test.

Final sizes of the combined training/inference containers for each framework:

Container	Tag	Size
merlin/base	22.05-pre	8.88GB
merlin/hugectr	22.05-pre	10.6GB
merlin/tf	22.05-pre	13.2GB
merlin/torch	22.05-pre	13.6GB

[github-actions]

Documentation preview

https://nvidia-merlin.github.io/Merlin/review/pr-234

[nvidia-merlin-bot]

Click to view CI Results

GitHub pull request #234 of commit 83307dba3e23fae9f1a68f30b840c44dc9d4369f, no merge conflicts.
Running as SYSTEM
Setting status of 83307dba3e23fae9f1a68f30b840c44dc9d4369f to PENDING with url https://10.20.13.93:8080/job/merlin_merlin/47/console and message: 'Pending'
Using context: Jenkins
Building on master in workspace /var/jenkins_home/workspace/merlin_merlin
using credential systems-login
 > git rev-parse --is-inside-work-tree # timeout=10
Fetching changes from the remote Git repository
 > git config remote.origin.url https://github.com/NVIDIA-Merlin/Merlin # timeout=10
Fetching upstream changes from https://github.com/NVIDIA-Merlin/Merlin
 > git --version # timeout=10
using GIT_ASKPASS to set credentials login for merlin-systems
 > git fetch --tags --force --progress -- https://github.com/NVIDIA-Merlin/Merlin +refs/pull/234/*:refs/remotes/origin/pr/234/* # timeout=10
 > git rev-parse 83307dba3e23fae9f1a68f30b840c44dc9d4369f^{commit} # timeout=10
Checking out Revision 83307dba3e23fae9f1a68f30b840c44dc9d4369f (detached)
 > git config core.sparsecheckout # timeout=10
 > git checkout -f 83307dba3e23fae9f1a68f30b840c44dc9d4369f # timeout=10
Commit message: "Add a container test for HugeCTR"
 > git rev-list --no-walk 9f88235c76a3e8eaf14142179b2422ba2737a9a6 # timeout=10
[merlin_merlin] $ /bin/bash /tmp/jenkins8788969305069239975.sh
============================= test session starts ==============================
platform linux -- Python 3.8.10, pytest-7.1.1, pluggy-1.0.0
rootdir: /var/jenkins_home/workspace/merlin_merlin/merlin
plugins: xdist-2.5.0, forked-1.4.0, cov-3.0.0
collected 1 item
tests/unit/test_version.py .                                             [100%]
============================== 1 passed in 0.01s ===============================

Performing Post build task...

Match found for : : True

Logical operation result is TRUE

Running script  : #!/bin/bash

cd /var/jenkins_home/

CUDA_VISIBLE_DEVICES=1 python test_res_push.py "https://api.GitHub.com/repos/NVIDIA-Merlin/Merlin/issues/$ghprbPullId/comments" "/var/jenkins_home/jobs/$JOB_NAME/builds/$BUILD_NUMBER/log"

[merlin_merlin] $ /bin/bash /tmp/jenkins4300395400609222406.sh