Switch swarm to using hyphens as a separator in hostnames

As far as I can tell, this has not been reported on this repository. It is a direct copy of https://github.com/docker/compose/issues/229 which was marked as closed on compose, but the exact same issue exists on docker stack deploy

Description

When using stack deploy, container names are prefixed with the stack name, then an underscore. The DNS entry for the container (regardless of vip or dnsrr) will be stackname_containername. This violates RFCs on valid hostnames (see linked ticket for RFC numbers). Apparently in the past multiple languages / webservers still allowed this, but in modern times multiple languages will reject any incoming HTTP request that has a Host header with an underscore. From the linked ticket, this problem exists in:

• Apache
• Spring Boot (via Tomcat)
• Ruby's URI parse method
• Django (via django.security.DisallowedHost)

Steps to reproduce the issue:

1. Launch any modern spring boot application as part of a stack
2. Exec into another container inside the same stack and curl/wget into that spring boot app via the DNS name

Describe the results you received:

For Tomcat, here is the resulting error

2021-11-17 01:38:07.226  INFO 1 --- [nio-8081-exec-2] o.apache.coyote.http11.Http11Processor   : The host [test_upstream-mock:8081] is not valid
 Note: further occurrences of request parsing errors will be logged at DEBUG level.

java.lang.IllegalArgumentException: The character [_] is never valid in a domain name.
	at org.apache.tomcat.util.http.parser.HttpParser$DomainParseState.next(HttpParser.java:974) ~[tomcat-embed-core-9.0.53.jar:9.0.53]

Describe the results you expected:

In https://github.com/docker/compose/issues/229 the decision was made to switch to using hyphens, instead of underscores, to prefix container names inside of a stack. These have no routing issues with various language components as they are valid and commonly used in URLs.

Of interest, this was apparently also part of the motivation for using periods in the suffix portion of swarm container naming (e.g. stackname_containername.1.pmybkzo8ovlirri0rf1syyo4p has periods for the number and id), but somehow the underscore for the prefix never went away even though the underscores that were (originally) used for the suffix values were removed

Output of docker version:

Client:
 Cloud integration: v1.0.20
 Version:           20.10.10
 API version:       1.41
 Go version:        go1.16.9
 Git commit:        b485636
 Built:             Mon Oct 25 07:43:15 2021
 OS/Arch:           darwin/amd64
 Context:           default
 Experimental:      true

Server: Docker Engine - Community
 Engine:
  Version:          20.10.10
  API version:      1.41 (minimum version 1.12)
  Go version:       go1.16.9
  Git commit:       e2f740d
  Built:            Mon Oct 25 07:41:30 2021
  OS/Arch:          linux/amd64
  Experimental:     false
 containerd:
  Version:          1.4.11
  GitCommit:        5b46e404f6b9f661a205e28d59c982d3634148f8
 runc:
  Version:          1.0.2
  GitCommit:        v1.0.2-0-g52b36a2
 docker-init:
  Version:          0.19.0
  GitCommit:        de40ad0

Output of docker info:

Client:
 Context:    default
 Debug Mode: false
 Plugins:
  buildx: Build with BuildKit (Docker Inc., v0.6.3)
  compose: Docker Compose (Docker Inc., v2.1.1)
  scan: Docker Scan (Docker Inc., 0.9.0)

Server:
 Containers: 7
  Running: 5
  Paused: 0
  Stopped: 2
 Images: 30
 Server Version: 20.10.10
 Storage Driver: overlay2
  Backing Filesystem: extfs
  Supports d_type: true
  Native Overlay Diff: true
  userxattr: false
 Logging Driver: json-file
 Cgroup Driver: cgroupfs
 Cgroup Version: 1
 Plugins:
  Volume: local
  Network: bridge host ipvlan macvlan null overlay
  Log: awslogs fluentd gcplogs gelf journald json-file local logentries splunk syslog
 Swarm: active
  NodeID: fb4iiubm41rqdw4xk6geclqop
  Is Manager: true
  ClusterID: ytub70qrgxhq1esdpeuypgdo3
  Managers: 1
  Nodes: 1
  Default Address Pool: 10.0.0.0/8
  SubnetSize: 24
  Data Path Port: 4789
  Orchestration:
   Task History Retention Limit: 5
  Raft:
   Snapshot Interval: 10000
   Number of Old Snapshots to Retain: 0
   Heartbeat Tick: 1
   Election Tick: 10
  Dispatcher:
   Heartbeat Period: 5 seconds
  CA Configuration:
   Expiry Duration: 3 months
   Force Rotate: 0
  Autolock Managers: false
  Root Rotation In Progress: false
  Node Address: 192.168.65.3
  Manager Addresses:
   192.168.65.3:2377
 Runtimes: io.containerd.runc.v2 io.containerd.runtime.v1.linux runc
 Default Runtime: runc
 Init Binary: docker-init
 containerd version: 5b46e404f6b9f661a205e28d59c982d3634148f8
 runc version: v1.0.2-0-g52b36a2
 init version: de40ad0
 Security Options:
  seccomp
   Profile: default
 Kernel Version: 5.10.47-linuxkit
 Operating System: Docker Desktop
 OSType: linux
 Architecture: x86_64
 CPUs: 8
 Total Memory: 5.804GiB
 Name: docker-desktop
 ID: EJRL:B3PO:RQI6:WXA7:UEHR:TTIN:3YK5:T4O6:3SYM:7JMB:7LEN:LD55
 Docker Root Dir: /var/lib/docker
 Debug Mode: false
 HTTP Proxy: http.docker.internal:3128
 HTTPS Proxy: http.docker.internal:3128
 Registry: https://index.docker.io/v1/
 Labels:
 Experimental: false
 Insecure Registries:
  127.0.0.0/8
 Live Restore Enabled: false

Start at the docker stack deploy entry point and trace how stack task and DNS names are generated. Reproduce the issue with a stack and a hostname containing an underscore, then verify that the generated prefix uses hyphens and that requests from another stack container reach the service successfully.