Fix deploy job; use absolute path for HOME

[glin]

Pulled out the smaller changes from https://github.com/rstudio/r-builds/pull/171 to fix the deploy job with the existing docker inside docker packager:

• Set HOME to an absolute path, env.WORKSPACE, rather than a relative ., which is needed for both the docker bind mounting to work correctly, and the serverless-python-requirements plugin correctly include Python packages in the zip file.
• Add docs for how the deploy job works
• Add verbose logging since the plugin hides the actual error on failures. Now you should be able to see the docker run command:

  [2023-07-06T23:37:22.224Z] ./node_modules/serverless/bin/serverless.js deploy --stage staging --verbose
  [2023-07-06T23:37:26.380Z] 
  [2023-07-06T23:37:26.380Z] Deploying r-builds to stage staging (us-east-1)
  [2023-07-06T23:37:26.380Z] 
  [2023-07-06T23:37:26.380Z] Generated requirements from /var/lib/jenkins/workspace/r-builds/deploy-r-builds/requirements.txt in /var/lib/jenkins/workspace/r-builds/deploy-r-builds/.serverless/requirements.txt
  [2023-07-06T23:37:26.380Z] Installing requirements from "/var/lib/jenkins/workspace/r-builds/deploy-r-builds/.cache/serverless-python-requirements/2b10c0467f75bfbcf12e9b77be65ec7563b90c5995f0eaa96faefbd42c337bdd_x86_64_slspyc/requirements.txt"
  [2023-07-06T23:37:26.380Z] Docker Image: lambci/lambda:build-python3.8
  [2023-07-06T23:37:26.380Z] Using download cache directory /var/lib/jenkins/workspace/r-builds/deploy-r-builds/.cache/serverless-python-requirements/downloadCacheslspyc
  [2023-07-06T23:37:26.380Z] Running docker run --rm -v /var/lib/jenkins/workspace/r-builds/deploy-r-builds/.cache/serverless-python-requirements/2b10c0467f75bfbcf12e9b77be65ec7563b90c5995f0eaa96faefbd42c337bdd_x86_64_slspyc\:/var/task\:z -v /var/lib/jenkins/workspace/r-builds/deploy-r-builds/.cache/serverless-python-requirements/downloadCacheslspyc\:/var/useDownloadCache\:z lambci/lambda\:build-python3.8 /bin/sh -c 'chown -R 0\\:0 /var/useDownloadCache && python3.8 -m pip install -t /var/task/ -r /var/task/requirements.txt --cache-dir /var/useDownloadCache && chown -R 386\\:386 /var/task && chown -R 386\\:386 /var/useDownloadCache'...

• Exclude other .cache/.npm generated files that didn't need to be in the serverless package

  $ unzip r-builds-staging-queueBuilds-fda4c5bf-5e39-4c62-9eef-e358b4135ceb.zip 
  $ du -sh $(ls -A) | sort -h | tail -n5
  3.8M    docutils
  28M .npm
  38M botocore
  46M r-builds-staging-queueBuilds-fda4c5bf-5e39-4c62-9eef-e358b4135ceb.zip
  56M .cache

I just tested this again to be sure at https://build.posit.it/blue/organizations/jenkins/r-builds%2Fdeploy-r-builds/detail/deploy-r-builds/202/pipeline/, and confirmed that staging builds still work.