Dockerfile: tweak to reduce the number of RUN and size

Re-order the commands to reduce the number of RUN to a single one.
It reduces the layers created.

The only code change is the addition of apt cache removal
(/var/lib/apt/lists/) to reduce the size from 158MB to 141MB.

Signed-off-by: Fathi Boudra <fathi.boudra@linaro.org>
5 jobs for <span class="ref-name">dockerfile-tweak</span> in 24 minutes and 20 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
passed #33413
amd64
debian-10

00:01:13

74.7%
 
  Analyze
passed #33414
amd64
black

00:00:15

passed #33415
amd64
codestyle

00:00:15

failed #33416
dind amd64 allowed to fail
sast

00:22:03

 
  Build
passed #33417
dind amd64
docker

00:01:03

 
Name Stage Failure
failed
sast Analyze
257e8c647a3e: Pull complete
Digest: sha256:30db5c0211cf38fd8e355782ef02931323a6bb62cd7543afa92249a1f2f109cd
Status: Downloaded newer image for registry.gitlab.com/gitlab-org/security-products/analyzers/find-sec-bugs-gradle:2
2019/03/15 17:11:03 Cannot connect to the Docker daemon at unix:///var/run/docker.sock. Is the docker daemon running?
2019/03/15 17:11:03 Timeout when running the analyzer
Uploading artifacts...
WARNING: gl-sast-report.json: no matching files
ERROR: No files to upload
ERROR: Job failed: exit code 1