Docs: Clarify interaction of USER with ENTRYPOINT

Description

This Pull Request updated the wording used in the documentation for the Dockerfile USER instruction.

The file was since moved to this repository in this commit

The new wording is not entirely accurate however. It states:

The USER instruction sets the user name (or UID) and optionally the user
group (or GID) to use when running the image and for any RUN, CMD and
ENTRYPOINT instructions that follow it in the Dockerfile.

However CMD and ENTRYPOINT commands run as the last USER command in the Dockerfile and don't care which USER command they follow.

I'm including a PR with suggested improved wording, comments are welcome.

Steps to reproduce

Dockerfile

FROM alpine:latest

USER root
RUN adduser -D test
ENTRYPOINT ["whoami"]

USER test

Run the resulting image with docker run

Expected output according to current docs: root
Actual output: test

Output of docker version:

Client:
 Version:           18.09.6
 API version:       1.39
 Go version:        go1.10.8
 Git commit:        481bc77
 Built:             Sat May  4 02:35:57 2019
 OS/Arch:           linux/amd64
 Experimental:      false

Server: Docker Engine - Community
 Engine:
  Version:          19.03.12
  API version:      1.40 (minimum version 1.12)
  Go version:       go1.13.10
  Git commit:       48a66213fe
  Built:            Mon Jun 22 15:49:35 2020
  OS/Arch:          linux/amd64
  Experimental:     false
 containerd:
  Version:          v1.2.13
  GitCommit:        7ad184331fa3e55e52b890ea95e65ba581ae3429
 runc:
  Version:          1.0.0-rc10
  GitCommit:        dc9208a3303feef5b3839f4323d9beb36df0a9dd
 docker-init:
  Version:          0.18.0
  GitCommit:        fec3683

Output of docker info:

Containers: 0
 Running: 0
 Paused: 0
 Stopped: 0
Images: 0
Server Version: 19.03.12
Storage Driver: overlay2
 Backing Filesystem: extfs
 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 local logentries splunk syslog
Swarm: inactive
Runtimes: runc
Default Runtime: runc
Init Binary: docker-init
containerd version: 7ad184331fa3e55e52b890ea95e65ba581ae3429
runc version: dc9208a3303feef5b3839f4323d9beb36df0a9dd
init version: fec3683
Security Options:
 seccomp
  Profile: default
Kernel Version: 4.19.130-boot2docker
Operating System: Boot2Docker 19.03.12 (TCL 10.1)
OSType: linux
Architecture: x86_64
CPUs: 1
Total Memory: 985.4MiB
Name: newest
ID: VLK7:FW5M:V5QL:OM4F:UXXY:UZWT:3WWC:ZB4A:HZWN:JGWE:HWJZ:UX6Z
Docker Root Dir: /mnt/sda1/var/lib/docker
Debug Mode (client): false
Debug Mode (server): false
Registry: https://index.docker.io/v1/
Labels:
 provider=virtualbox
Experimental: false
Insecure Registries:
 127.0.0.0/8
Live Restore Enabled: false
Product License: Community Engine

Additional environment details (AWS, VirtualBox, physical, etc.):
NA

Locate the current Dockerfile documentation entry for USER and compare its wording with the linked pull request and the reproduction example. Update the explanation so it accurately describes how USER affects RUN, CMD, and ENTRYPOINT, then verify that the documented behavior matches the example.