ci: add support for docker in jenkins

Jenkinsfile

@@ -0,0 +1,29 @@
+
+pipeline {
+    agent any
+
+    stages {
+        stage('Build') {
+    		agent {
+                docker {
+                    image 'crystallang/crystal:1.11.0-alpine'
+                    reuseNode true
+                }
+            }
+            steps {
+                sh 'shards install'
+                sh 'shards build --production --static'
+            }
+        }
+        stage('Test') {
+            steps {
+                echo 'Testing..'
+            }
+        }
+        stage('Deploy') {
+            steps {
+                echo 'Deploying....'
+            }
+        }
+    }
+}