feat: build and push images in GitLab

Closes #122 (closed), #125 (closed)

Builds and pushes the docker images if a tag is created. Uses .build_and_push_image which relies on DOCKERFILE, DOCKER_CONTEXT and IMAGE_NAME to be defined. If either is undefined, the pipeline will fail and notify (see example).

Does not build the nginx image, because we use the default nginx image with customised nginx.conf in our deployment repository.

For the working pipeline see here. This pipeline was manually created and the variable CI_COMMIT_TAG was set to test.