Revert "duneci-build-docker-image: set DUNECI_PARALLEL when building images"

This reverts commit e0e38897.
15 jobs for master in 25 minutes and 39 seconds (queued for 1 second)
Status Job ID Name Coverage
  Stage0
passed #3492
pull base distributions

00:00:16

 
  Stage1
passed #3496
base:10

00:00:12

passed #3497
base:16.04

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passed #3498
base:16.10

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passed #3494
base:8

00:00:11

passed #3495
base:9

00:00:12

passed #3493
docker-builder

00:00:11

 
  Stage2
passed #3499
base:8-backports

00:00:11

passed #3500
dune:2.3

00:00:12

passed #3501
dune:2.4

00:00:12

passed #3502
dune:2.5

00:00:24

passed #3503
dune:git

00:15:59

 
  Stage3
passed #3504
dune-fufem:2.4

00:03:46

passed #3505
dune-fufem:git

00:03:24

 
  Publish
passed #3506
publish

00:04:59