fix invalid docker tag in CI builds

Former-commit-id: f6386210

.gitlab-ci.yml

@@ -85,7 +85,7 @@ build:packages:
 build:docker:
   stage: build
   script:
-    - docker build -f Packaging/Docker/Dockerfile -t ${DOCKER_IMAGE}:${DOCKER_TAG} .
+    - docker build -f Packaging/Docker/Dockerfile --build-arg DOCKER_TAG=${DOCKER_TAG} -t ${DOCKER_IMAGE}:${DOCKER_TAG} .
   tags:
     - shell
 

Packaging/Docker/Dockerfile

 # rwthacs/dpsim-dev is built by dpsim-git/Packaging/Docker/Dockerfile.dev
-FROM rwthacs/dpsim-dev AS builder
+ARG DOCKER_TAG=latest
+FROM rwthacs/dpsim-dev:${DOCKER_TAG} AS builder
 
 COPY . /dpsim/