ci: add basic jenkinsfile for building Docker images

This is the first and most basic version of a Jenkinsfile we can use.

Since we can't just hardcode things like docker name, tag, or
credentials for pushing to a Docker registry it needs to be a parameter.

Signed-off-by: Jakub Sokołowski <jakub@status.im>

Jenkinsfile

@@ -0,0 +1,70 @@
+pipeline {
+  agent { label 'linux' }
+
+  options {
+    timestamps()
+    timeout(time: 20, unit: 'MINUTES')
+    disableConcurrentBuilds()
+    buildDiscarder(logRotator(
+      numToKeepStr: '10',
+      daysToKeepStr: '30',
+    ))
+  }
+
+  parameters {
+    string(
+      name: 'COMPONENT',
+      description: 'Name of component to build.',
+      defaultValue: params.COMPONENT ?: 'backend'
+    )
+    string(
+      name: 'DOCKER_TAG',
+      description: 'Name of Docker tag to push. Chose wisely.',
+      defaultValue: params.DOCKER_TAG ?: 'latest',
+    )
+    string(
+      name: 'DOCKER_NAME',
+      description: 'Name of Docker image to push.',
+      defaultValue: params.DOCKER_NAME ?: 'ghcr.io/sartography/spiffworkflow-backend',
+    )
+    string(
+      name: 'DOCKER_CRED_ID',
+      description: 'ID of Jenkins credential for Docker registry.',
+      defaultValue: params.DOCKER_CRED_ID ?: 'MISSING'
+    )
+    booleanParam(
+      name: 'PUBLISH',
+      description: 'Publish built Docker images.',
+      defaultValue: params.PUBLISH ?: false
+    )
+  }
+
+  stages {
+    stage('Build') {
+      steps { script {
+        dir("spiffworkflow-${params.COMPONENT}") {
+          image = docker.build(
+            "${params.DOCKER_NAME}:${env.GIT_COMMIT.take(8)}",
+            "--label=commit='${env.GIT_COMMIT.take(8)}' ."
+          )
+        }
+      } }
+    }
+
+    stage('Push') {
+      when { expression {
+        params.PUBLISH && params.DOCKER_CRED_ID != ''
+      } }
+      steps { script {
+        withDockerRegistry([credentialsId: params.DOCKER_CRED_ID, url: ""]) {
+          image.push()
+          image.push(env.DOCKER_TAG)
+        }
+      } }
+    }
+  } // stages
+  post {
+    always  { sh 'docker image prune -f' }
+    cleanup { cleanWs() }
+  } // post
+} // pipeline