Add Ulimits to ContainerSpec

[akerouanton]

- What I did

Unlike Docker and docker-compose, Swarm never supported ulimits.

This change introduces a new Ulimits field on the ContainerSpec type. It
can be used by API clients to set desired ulimits.

This is related to moby/moby#40639.

- How to test it

term1$ sudo ./bin/swarmd
term2$ sudo ./bin/swarmctl service create --ulimit=nofile=100 --image debian --command /bin/bash --args="-c" --args="ulimit -n" --name test
term2$ sudo ./bin/swarmctl service logs -f test
test.1@aker❯ 100

- Description for the changelog

Add Ulimits field to the API

[codecov]

Codecov Report

Merging #2967 into master will increase coverage by 0.08%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2967      +/-   ##
==========================================
+ Coverage   61.75%   61.84%   +0.08%     
==========================================
  Files         142      142              
  Lines       22999    23005       +6     
==========================================
+ Hits        14204    14228      +24     
+ Misses       7295     7278      -17     
+ Partials     1500     1499       -1     

[thaJeztah]

Whoops! looks like we missed this in #2965)

I guess the linter did not complain about this because it skips the generated .pb.go files 🤔

Could you perhaps create a separate PR for this change?

[akerouanton]

I reverted the change here and created #2968 😉

[thaJeztah]

@cpuguy83 @dperny PTAL

[dperny]

This looks fine. Swarm is just plumbing this field down, and the hard work is all handled in the Engine.

Nota bene, @akerouanton, that the github.com/swarmkit/agent/exec/dockerapi package is only used for internally testing swarmkit, and you'll need to make identical changes to the github.com/docker/docker/daemon/cluster/executor package.