Remove HOME, restore jenkins user; reenable plugin caching; update comments

* Simplify the deployment as much as possible
* HOME can be removed, but needs the jenkins user restored to have a
  valid HOME dir that's not /, which breaks npm.

Author: glin

Makefile

@@ -30,18 +30,11 @@ ecr-login:
 push-serverless-custom-file:
 	aws s3 cp serverless-custom.yml s3://rstudio-devops/r-builds/serverless-custom.yml
 
-# Temporarily patch serverless-custom.yml to not Dockerize pip and disable
-# serverless-python-requirements caching. The caching does not work in Jenkins
-# by default because the cache directory is resolved to a relative ".cache"
-# directory, rather than an absolute directory, which breaks the plugin.
-# The cache directory must either be an absolute path (optionally specified
-# via cacheLocation: /path/to/cache), or caching must be disabled for the plugin
-# to work. We disable caching completely to avoid all sorts of future issues.
-# The cache directory is based on $HOME, which is set to "." when running
-# Docker images in Jenkins for some reason.
+# Temporarily patch serverless-custom.yml to not Dockerize pip for the
+# serverless-python-requirements plugin.
 fetch-serverless-custom-file:
 	aws s3 cp s3://rstudio-devops/r-builds/serverless-custom.yml .
-	sed -i 's|dockerizePip: true|dockerizePip: false\n  useStaticCache: false\n  useDownloadCache: false|' serverless-custom.yml
+	sed -i 's|dockerizePip: true|dockerizePip: false|' serverless-custom.yml
 
 rebuild-all: deps fetch-serverless-custom-file
 	$(SLS_BINARY) invoke stepf -n rBuilds -d '{"force": true}'

deploy.Jenkinsfile

@@ -5,13 +5,6 @@ pipeline {
       label 'docker'
     }
   }
-  environment {
-    // Set HOME to the workspace for the serverless-python-requirements plugin.
-    // The plugin uses HOME for its default cache location, which needs to be
-    // mounted in the separate Python build container. This needs to be an absolute
-    // path that also exists on the host since we're mounting the Docker socket.
-    HOME = "${env.WORKSPACE}"
-  }
   options {
     ansiColor('xterm')
   }

jenkins/Dockerfile

@@ -5,6 +5,9 @@
 
 FROM ubuntu:jammy
 
+ARG JENKINS_UID=386
+ARG JENKINS_GID=386
+
 # Keep the Python version in line with the runtime in serverless.yml
 ARG PYTHON_VERSION=3.8.15
 
@@ -32,3 +35,9 @@ RUN curl -fsSL https://deb.nodesource.com/setup_16.x | bash - \
   && apt install -y nodejs
 
 RUN npm install -g serverless
+
+# Create the jenkins user with the same id:gid as the jenkins node
+RUN groupadd -g $JENKINS_GID jenkins \
+  && useradd -m -d /var/lib/jenkins -u $JENKINS_UID -g jenkins jenkins
+
+USER jenkins