Rework cpuShares into cpuQuota

cpuShares is the relative weight wrt other apps. This is used when
there is contention for CPU. If we want this, maybe we implement
a UI where we show all the apps and let the user re-order them.
As it stands, it is confusing.

cpuQuota is a more straightforward "hard limit" of the CPU% that you
want the app to consume.

Can be tested with : stress -c 8 -t 20s

src/docker.js

@@ -42,6 +42,7 @@ const apps = require('./apps.js'),
     debug = require('debug')('box:docker'),
     Docker = require('dockerode'),
     fs = require('fs'),
+    os = require('os'),
     paths = require('./paths.js'),
     promiseRetry = require('./promise-retry.js'),
     services = require('./services.js'),
@@ -357,7 +358,9 @@ async function createSubcontainer(app, name, cmd, options) {
                 'Name': isAppContainer ? 'unless-stopped' : 'no',
                 'MaximumRetryCount': 0
             },
-            CpuShares: app.cpuShares,
+            // CpuPeriod (100000 microseconds) and CpuQuota(app.cpuQuota% of CpuPeriod)
+            // 1000000000 is one core https://github.com/moby/moby/issues/24713#issuecomment-233167619 and https://stackoverflow.com/questions/52391877/set-the-number-of-cpu-cores-of-a-container-using-docker-engine-api
+            NanoCPUs: app.cpuQuota === 100 ? 0 : (os.cpus().length * app.cpuQuota/100).toFixed(2) * 1000000000,
             VolumesFrom: isAppContainer ? null : [ app.containerId + ':rw' ],
             SecurityOpt: [ 'apparmor=docker-cloudron-app' ],
             CapAdd: [],