Merge branch 'main' into pr/223

* main:
  Simplify domain language to just "boot" and unscoped config keys
  Retain a fixed number of containers when pruning
  Don't assume rolling back in message
  Check all hosts before rolling back
  Ensure Traefik service name is consistent
  Extend traefik delay by 1 second
  Include traefik access logs
  Check if we are still getting a 404
  Also dump load balancer logs
  Dump traefik logs when app not booted
  Fix missing for apt-get
  Report on container health after failure
  Fix the integration test healthcheck
  Allow percentage-based rolling deployments
  Move `group_limit` & `group_wait` under `boot`
  Limit rolling deployment to boot operation
  Allow performing boot & start operations in groups

README.md

@@ -864,6 +864,24 @@ mrsk lock acquire -m "Doing maintanence"
 mrsk lock release
 ```
 
+## Rolling deployments
+
+When deploying to large numbers of hosts, you might prefer not to restart your services on every host at the same time.
+
+MRSK's default is to boot new containers on all hosts in parallel. But you can control this by configuring `boot/limit` and `boot/wait` as options:
+
+```yaml
+service: myservice
+
+boot:
+  limit: 10 # Can also specify as a percentage of total hosts, such as "25%"
+  wait: 2
+```
+
+When `limit` is specified, containers will be booted on, at most, `limit` hosts at once. MRSK will pause for `wait` seconds between batches.
+
+These settings only apply when booting containers (using `mrsk deploy`, or `mrsk app boot`). For other commands, MRSK continues to run commands in parallel across all hosts.
+
 ## Stage of development
 
 This is beta software. Commands may still move around. But we're live in production at [37signals](https://37signals.com).