CI: add --no-cache option to docker build, to clear out stale cache

.gitlab-ci.yml

@@ -29,7 +29,7 @@ gwinc/base:
       RUN apt-get update -qq
       RUN apt-get -y install --no-install-recommends git python3 python3-gitlab python3-setuptools-scm python3-yaml python3-scipy python3-matplotlib python3-pypdf2 python3-h5py python3-inspiral-range python3-lalsimulation
       EOF
-    - docker build -t $IMAGE_TAG .
+    - docker build --no-cache -t $IMAGE_TAG .
     - docker push $IMAGE_TAG
 
 # create plots for the canonical IFOs