Use Debian Buster in stats dockerfile (is already used in backend and...

Use Debian Buster in stats dockerfile (is already used in backend and frontend, so this saves space)
4 jobs for master in 22 minutes and 20 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #65591
test

00:04:12

 
  Build
passed #65592
backend

00:13:20

passed #65593
frontend

00:03:59

passed #65594
stats

00:00:47