Try running with Dockerfile

Dockerfile

@@ -0,0 +1,5 @@
+FROM silex/emacs:27.1-alpine-dev
+
+RUN git clone https://github.com/cask/cask.git /usr/local/cask
+ENV PATH="/usr/local/cask/bin:$PATH"
+RUN cask upgrade-cask

Jenkinsfile

@@ -1,17 +1,13 @@
-def dockerImage = 'silex/emacs:27.1-alpine-dev'
+pipeline {
+    agent { dockerfile true }
 
-node('docker') {
-    checkout scm
-
-    stage('Build') {
-        docker.image(dockerImage).inside {
+    stages {
+        stage('Build') {
             sh 'make'
             stash includes: 'public/*', name: 'public_html'
         }
-    }
 
-    stage('Deploy') {
-        docker.image('instrumentisto/rsync-ssh').inside {
+        stage('Deploy') {
             unstash 'public_html'
 
             withCredentials([file(credentialsId: 'ryuslash.org-deploy-key', variable: 'KEY_FILE')]) {