refactor(ci): set git describe as build number

Jenkinsfile

@@ -35,7 +35,7 @@ pipeline {
 
         stage('compile') {
             steps {
-                sh 'mvn clean package'
+                sh "BUILD_NUMBER=${version} mvn clean package"
             }
         }
 

pom.xml

@@ -4,7 +4,7 @@
 
     <groupId>de.hftstuttgart</groupId>
     <artifactId>modocot-backend</artifactId>
-    <version>2.0.0</version>
+    <version>${buildNumber}</version>
     <packaging>jar</packaging>
 
     <name>Modocot-Backend</name>
@@ -17,11 +17,24 @@
     </parent>
 
     <properties>
+        <buildNumber>devel</buildNumber>
         <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
         <project.reporting.outputEncoding>UTF-8</project.reporting.outputEncoding>
         <java.version>11</java.version>
     </properties>
 
+    <profiles>
+        <profile>
+            <id>ci</id>
+            <activation>
+                <property><name>env.BUILD_NUMBER</name></property>
+            </activation>
+            <properties>
+                <buildNumber>${env.BUILD_NUMBER}</buildNumber>
+            </properties>
+        </profile>
+    </profiles>
+
     <dependencies>
         <dependency>
             <groupId>com.github.docker-java</groupId>