Explain that single quotes will escape everything in LABELs

Description
Using variables in single quotes of a LABEL instruction will cause it to not be expanded. This seems to be the expected behaviour from shells that has been translated into a Dockerfile. Alright, no big deal I suppose. But can we get this mentioned in the builder.md file?

Steps to reproduce the issue:

FROM scratch
ARG arg="abc"
ENV env="def"
LABEL abc='$arg'
LABEL def='$env'

$ docker build .
Sending build context to Docker daemon  1.571MB
Step 1/5 : FROM scratch
 --->
Step 2/5 : ARG arg="abc"
 ---> Running in ba8af669a2d2
 ---> f88b7049ae5e
Removing intermediate container ba8af669a2d2
Step 3/5 : ENV env "def"
 ---> Running in 0c185027596d
 ---> 9ced66814d7a
Removing intermediate container 0c185027596d
Step 4/5 : LABEL abc '$arg'
 ---> Running in 28d867245926
 ---> ff415136967f
Removing intermediate container 28d867245926
Step 5/5 : LABEL def '$env'
 ---> Running in 05e1fca053cb
 ---> 9d3437a0c8cc
Removing intermediate container 05e1fca053cb
Successfully built 9d3437a0c8cc

$ docker inspect 9d3437a0c8cc
"Labels": {
  "abc": "$arg",
  "def": "$env"
}

Output of docker version:

$ docker --version
Docker version 17.09.0-ce, build afdb6d4

Output of docker info:

$ docker info
Containers: 0
 Running: 0
 Paused: 0
 Stopped: 0
Images: 4
Server Version: 17.09.0-ce
Storage Driver: overlay2
 Backing Filesystem: xfs
 Supports d_type: true
 Native Overlay Diff: true
Logging Driver: json-file
Cgroup Driver: cgroupfs
Plugins:
 Volume: local
 Network: bridge host ipvlan macvlan null overlay
 Log: awslogs fluentd gcplogs gelf journald json-file logentries splunk syslog
Swarm: inactive
Runtimes: runc
Default Runtime: runc
Init Binary: docker-init
containerd version: 06b9cb35161009dcb7123345749fef02f7cea8e0
runc version: 3f2f8b84a77f73d38244dd690525642a72156c64
init version: 949e6fa
Security Options:
 apparmor
 seccomp
  Profile: default
Kernel Version: 4.4.0-96-generic
Operating System: Alpine Linux v3.6 (containerized)
OSType: linux
Architecture: x86_64
CPUs: 8
Total Memory: 31.4GiB
Name: node1
ID: O7HP:AR4C:YRH3:DNIQ:HGF4:5FRX:VHTJ:73Z6:DZ4J:LDDH:VTOF:3732
Docker Root Dir: /var/lib/docker
Debug Mode (client): false
Debug Mode (server): true
 File Descriptors: 18
 Goroutines: 27
 System Time: 2017-11-13T21:46:57.229067696Z
 EventsListeners: 0
Registry: https://index.docker.io/v1/
Experimental: true
Insecure Registries:
 127.0.0.1
 127.0.0.0/8
Live Restore Enabled: false

WARNING: No swap limit support
WARNING: bridge-nf-call-iptables is disabled
WARNING: bridge-nf-call-ip6tables is disabled

Additional environment details (AWS, VirtualBox, physical, etc.):
Tested via PWD.

Open docs/reference/builder.md and review the LABEL instruction documentation alongside the provided single-quoted examples. Explain that variables inside single quotes are not expanded, then verify that the wording accurately matches the demonstrated docker build and inspect behavior.