Update Docker version to 18.09.3

[joakimr-axis]

Change-Id: I1ac2a617f03443b9f7327dcddcde4af8275e32e0
Signed-off-by: Joakim Roubert joakimr@axis.com

Resolves #6560

[GordonTheTurtle]

Please sign your commits following these rules:
https://github.com/moby/moby/blob/master/CONTRIBUTING.md#sign-your-work
The easiest way to do this is to amend the last commit:

$ git clone -b "updatedocker" git@github.com:joakimr-axis/compose.git somewhere
$ cd somewhere
$ git rebase -i HEAD~842357817216
editor opens
change each 'pick' to 'edit'
save the file and quit
$ git commit --amend -s --no-edit
$ git rebase --continue # and repeat the amend for each commit
$ git push -f

Amending updates the existing PR. You DO NOT need to open a new one.

[tbeadle]

+1 This would be good for us so that we can easily use buildkit from within the docker-compose image.

[thaJeztah]

Note that 18.09.4 is now released; alternatively, use 18.09, which would always be the latest patch release of Docker 18.09

[joakimr-axis]

18.09.3 was the latest release when this was created. If I update it to 18.09.4 and nothing happens still with the patch we will see 18.09.5 or something superseding. Using 18.09 as suggested would implicate I could not actually guarantee that my patch was tested with the version that 18.09 is currently pointing towards. Perhaps the latter is not considered an issue.
I will see if any of the maintainers step in and have suggestions on how they would like this (if they do at all).

[chris-crone]

Thanks for the PR @joakimr-axis.

@ulyssessouza is reworking the Dockerfiles for this project so this is superseded by: #6641