Hard links of files don't invalidate the cache

[jgouly]

Description

If a file that is a hard link is ADDed into a container, the cache is not invalidated when the contents change.

Steps to reproduce the issue:

$ touch a
$ ln a b
$ cat Dockerfile
FROM busybox
ADD b b
CMD md5sum b
$ docker build . -t s && docker run s && md5sum a b
$ # modify a
$ docker build . -t s && docker run s && md5sum a b # this does not rebuild

Describe the results you received:

The container didn't rebuild / invalidate the cache.

Describe the results you expected:

The container to be rebuilt.

Output of docker version:

Docker version 18.09.1, build 4c52b90

Output of docker info:

docker info
Containers: 62
 Running: 0
 Paused: 0
 Stopped: 62
Images: 89
Server Version: 18.09.1
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 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: 9754871865f7fe2f4e74d43e2fc7ccd237edcbce
runc version: 1.0.0~rc6+dfsg1-3
init version: v0.18.0 (expected: fec3683b971d9c3ef73f284f176672c44b448662)
Security Options:
 apparmor
 seccomp
  Profile: default
Kernel Version: 4.19.0-16-amd64
Operating System: Debian GNU/Linux 10 (buster)
OSType: linux
Architecture: x86_64
CPUs: 1
Total Memory: 483.4MiB
Name: debian
ID: 6S6O:HJAF:IOOU:LCMR:SETE:OJ6V:4TJ6:7SR3:GLJY:QERN:M5YK:6BOT
Docker Root Dir: /var/lib/docker
Debug Mode (client): false
Debug Mode (server): false
Registry: https://index.docker.io/v1/
Labels:
Experimental: false
Insecure Registries:
 127.0.0.0/8
Live Restore Enabled: false

WARNING: No swap limit support