Set the ubuntu release explicitly in the docker image otherwise it breaks the CI!

4 jobs for master in 21 minutes and 50 seconds (queued for 2 seconds)
Name Stage Failure
expe-dockver Test
gg-deco "" ../ -o
make[2]: Leaving directory '/builds/verimag/reproducible-research/sasartifact/expe/async-unison'
make[1]: Leaving directory '/builds/verimag/reproducible-research/sasartifact/expe'
/bin/sh: 1: gg-deco: not found
make[2]: *** [Makefile.untracked:4:] Error 127
make[1]: *** [Makefile:61: async-unison.expe] Error 2
make: *** [Makefile:4: all] Error 2
Cleaning up file based variables
ERROR: Job failed: exit code 1