Add Jenkins pipeline for image publish

Reviewed-on: #1

Jenkinsfile

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+pipeline {
+    agent any
+
+    environment {
+        GIT_URL = 'https://gitea.mindboost.team/mindboost/education-flagger.git'
+        GIT_BRANCH = 'pipeline/deploy-image'
+        REGISTRY_SCHEME = 'https'
+        REGISTRY_AUTHORITY = 'gitea.mindboost.team'
+        IMAGE_NAME = 'mindboost/education-flagger'
+        REGISTRY_CREDENTIALS_ID = 'REGISTRY_CREDENTIALS_ID'
+    }
+
+    stages {
+        stage('Checkout') {
+            steps {
+                script {
+                    checkout([
+                        $class: 'GitSCM',
+                        branches: [[name: "*/${env.GIT_BRANCH}"]],
+                        userRemoteConfigs: [[
+                            url: env.GIT_URL,
+                            credentialsId: 'b5f383be-8c74-40f9-b7e1-3a9c5856df0e'
+                        ]]
+                    ])
+                }
+            }
+        }
+
+        stage('Check Repository') {
+            steps {
+                script {
+                    sh 'pwd'
+                    sh 'ls -la'
+                    sh 'git status'
+                }
+            }
+        }
+
+        stage('Determine Version') {
+            steps {
+                script {
+                    def fullHash = sh(
+                        script: 'git rev-parse HEAD',
+                        returnStdout: true
+                    ).trim()
+                    env.IMAGE_TAG = "sha256-${fullHash}"
+                    echo "Resolved image tag: ${env.IMAGE_TAG}"
+                }
+            }
+        }
+
+        stage('Check Docker Image with the same tag') {
+            steps {
+                script {
+                    def imageExists = sh(
+                        script: "docker images -q ${env.IMAGE_NAME}:${env.IMAGE_TAG} || true",
+                        returnStdout: true
+                    ).trim()
+
+                    if (imageExists) {
+                        echo "Docker Image mit Tag ${env.IMAGE_TAG} existiert bereits. Überspringe Build."
+                        currentBuild.result = 'SUCCESS'
+                        return
+                    } else {
+                        echo "Kein vorhandenes Docker Image gefunden. Baue neues Image..."
+                    }
+                }
+            }
+        }
+
+        stage('Build Docker Image') {
+            when {
+                expression { currentBuild.result == null }
+            }
+            steps {
+                script {
+                    sh "docker build --rm -t ${env.IMAGE_NAME}:${env.IMAGE_TAG} ."
+                }
+            }
+        }
+
+        stage('Push Docker Image') {
+            when {
+                expression { currentBuild.result == null }
+            }
+            steps {
+                script {
+                    withCredentials([usernamePassword(
+                        credentialsId: env.REGISTRY_CREDENTIALS_ID,
+                        usernameVariable: 'REGISTRY_USER',
+                        passwordVariable: 'REGISTRY_PASS'
+                    )]) {
+                        def registryEndpoint = "${env.REGISTRY_SCHEME}://${env.REGISTRY_AUTHORITY}"
+                        sh "echo '${REGISTRY_PASS}' | docker login ${env.REGISTRY_AUTHORITY} -u '${REGISTRY_USER}' --password-stdin"
+                        sh "docker tag ${env.IMAGE_NAME}:${env.IMAGE_TAG} ${env.REGISTRY_AUTHORITY}/${env.IMAGE_NAME}:${env.IMAGE_TAG}"
+                        sh "docker push ${env.REGISTRY_AUTHORITY}/${env.IMAGE_NAME}:${env.IMAGE_TAG}"
+                        sh "docker logout ${env.REGISTRY_AUTHORITY}"
+                    }
+                }
+            }
+        }
+
+        stage('Cleanup Docker Images') {
+            steps {
+                script {
+                    sh 'set -eux; docker image prune -f; docker builder prune -f'
+                }
+            }
+        }
+    }
+}