use Docker image matching CI branch

2 jobs for master in 22 minutes and 52 seconds (queued for 11 minutes and 36 seconds)
Status Job ID Name Coverage
  Build
failed #17962
fp
build-coq.8.6.1

00:16:07

passed #17961
fp
build-coq.8.7.2

00:15:53

 
Name Stage Failure
failed
build-coq.8.6.1 Build
COQC theories/examples/symbol.v
make[1]: Target 'all' not remade because of errors.
Makefile:6: recipe for target 'all' failed
make: *** [all] Error 2

real 5m15.460s
user 13m39.056s
sys 0m20.324s
ERROR: Job failed: exit code 1