Changes to Dockerfile and Jenkinsfile

1. Install Catch2 from sources inside a Docker stage/layer
2. Some tweaks to Jenkinsfile

automation/Dockerfile

@@ -6,3 +6,9 @@ RUN apt-get --yes upgrade
 #tzdata is a dependency, won't install otherwise
 ARG DEBIAN_FRONTEND=noninteractive
 RUN apt-get --yes install gcc g++ cmake make lcov git valgrind nano
+
+RUN git clone https://github.com/catchorg/Catch2.git && \
+	cd Catch2 && \
+	git checkout v3.0.0-preview4 && \
+	cmake -Bbuild -H. -DBUILD_TESTING=OFF && \
+	cmake --build build/ --target install

automation/Jenkinsfile

@@ -1,22 +1,28 @@
 pipeline {
     agent any
     environment {
+        DOCKER_TAG = 'fsfw-build'
         BUILDDIR = 'build-tests'
     }
     stages {
-        stage('Create Docker') {
-            agent {
-                dockerfile {
-                    dir 'automation'
-                    additionalBuildArgs  '--no-cache'
-                    reuseNode true
+        stage('Build Container') {
+            when {
+                anyOf {
+                    changeset "automation/Dockerfile";
+                    changelog 'cleanDocker'
                 }
             }
             steps {
-                sh 'rm -rf $BUILDDIR'
+                sh 'docker build -t $DOCKER_TAG - < automation/Dockerfile'
             }
         }
         stage('Clean') {
+            agent {
+                docker {
+                    image '$DOCKER_TAG'
+                    reuseNode true
+                }
+            }
             when {
                 anyOf {
                     changelog 'cleanCI'
@@ -30,8 +36,8 @@ pipeline {
         }
         stage('Configure') {
             agent {
-                dockerfile {
-                    dir 'automation'
+                docker {
+                    image '$DOCKER_TAG'
                     reuseNode true
                 }
             }
@@ -43,8 +49,8 @@ pipeline {
         }
         stage('Build') {
             agent {
-                dockerfile {
-                    dir 'automation'
+                docker {
+                    image '$DOCKER_TAG'
                     reuseNode true
                 }
             }
@@ -56,8 +62,8 @@ pipeline {
         }
         stage('Unittests') {
             agent {
-                dockerfile {
-                    dir 'automation'
+                docker {
+                    image '$DOCKER_TAG'
                     reuseNode true
                 }
             }
@@ -69,8 +75,8 @@ pipeline {
         }
         stage('Valgrind') {
             agent {
-                dockerfile {
-                    dir 'automation'
+                docker {
+                    image '$DOCKER_TAG'
                     reuseNode true
                 }
             }