Fix deploy job; use absolute path for HOME

deploy.Jenkinsfile

@@ -3,12 +3,18 @@ pipeline {
     dockerfile {
       dir 'jenkins'
       label 'docker'
+      // Required for the serverless-python-requirements plugin to install
+      // Python requirements using the AWS build images, running Docker inside Docker.
       additionalBuildArgs '--build-arg DOCKER_GID=$(stat -c %g /var/run/docker.sock) --build-arg JENKINS_UID=$(id -u jenkins) --build-arg JENKINS_GID=$(id -g jenkins)'
       args '-v /var/run/docker.sock:/var/run/docker.sock --group-add docker'
     }
   }
   environment {
-    HOME = "."
+    // Set HOME to the workspace for the serverless-python-requirements plugin.
+    // The plugin uses HOME for its default cache location, which needs to be
+    // mounted in the separate Python build container. This needs to be an absolute
+    // path that also exists on the host since we're mounting the Docker socket.
+    HOME = "${env.WORKSPACE}"
   }
   options {
     ansiColor('xterm')