Fix CI

Closes: #66 (closed)

@bbb attempting to fix CI after I obviously messed it up in !81 (merged)

.gitlab-ci.yml

@@ -128,6 +128,7 @@ validate-classic:
 
 .doc: 
   stage: process
+  image: mathcomp/mathcomp:1.10.0-coq-8.11
   script:
     - make html -j ${NJOBS}
     - mv html with-proofs
@@ -166,6 +167,7 @@ doc-classic:
 
 proof-state:
   stage: process
+  image: mathcomp/mathcomp:1.10.0-coq-8.11
   needs: ["build-for-process"]
   dependencies:
     - build-for-process