fixes dockerfile building on recent linux installations

[Narfinger]

The current dockerfile in the root directory does not work on linux with systemd>=227.
It aborts with some errors about not being able to get nuget packages.

The solution is to either allow more tasks to run by systemd or disable parallel building as given in these two resources.

• https://success.docker.com/article/how-to-reserve-resource-temporarily-unavailable-errors-due-to-tasksmax-setting
• https://stackoverflow.com/questions/59476800/resource-temporarily-unavailable-when-building-dotnet-core-3-0-dependency-on-ubu

[EraYaN]

Ooh damn, systemd never really stops disappointing...

EDIT:
The real solution is this one: sudo systemctl set-property docker.service TasksMax=infinity (replace infinity with whatever is high enough, we don't have THAT may jobs)

[dkanada]

@Narfinger the issue is apparently something else entirely according to chat. The image is apparently incorrect for the ARM flavors after an upstream change.

[Narfinger]

I am fine if this does not get merged but perhaps there should be some information about why the current dockerfile does not compile out of the box on modern systems. Especially because it breaks with very useless messages.

I don't understand, however, how this could break arm builds.

[dkanada]

This specific issue doesn't break ARM builds. It's another issue entirely, and I think your pull request isn't related to that at all. The timing made me think you were trying to fix the nightly build issue.

[JustAMan]

Should be harmless (only slightly longer builds)...