CLI should ignore .DS_Store files in context bundles

Description

I recently looked at one of my context bundles' TLS files in Finder on macOS, which created a .DS_Store file. The Finder stores view metadata in .DS_Store files. The Docker CLI then complained on every command invocation about the file.

Steps to reproduce the issue:

1. Open a context's TLS directory in the Finder.
2. Do something like change the window view settings to cause a .DS_Store to be created.

Describe the results you received:

When the .DS_Store file is present, every CLI invocation printed a warning about an unknown file.

WARN[0000] unknown file .DS_Store in context sa-processors-ext tls bundle 

Describe the results you expected:

The CLI should ignore .DS_Store files.

Additional information you deem important (e.g. issue happens only occasionally):

N/A

Output of docker version:

Client: Docker Engine - Community
 Version:           19.03.2
 API version:       1.40
 Go version:        go1.12.8
 Git commit:        6a30dfc
 Built:             Thu Aug 29 05:26:49 2019
 OS/Arch:           darwin/amd64
 Experimental:      true

Server: Docker Engine - Community
 Engine:
  Version:          19.03.3
  API version:      1.40 (minimum version 1.12)
  Go version:       go1.12.10
  Git commit:       a872fc2f86
  Built:            Tue Oct  8 00:56:46 2019
  OS/Arch:          linux/amd64
  Experimental:     false
 containerd:
  Version:          1.2.6
  GitCommit:        894b81a4b802e4eb2a91d1ce216b8817763c29fb
 runc:
  Version:          1.0.0-rc8
  GitCommit:        425e105d5a03fabd737a126ad93d62a9eeede87f
 docker-init:
  Version:          0.18.0
  GitCommit:        fec3683

Output of docker info:

N/A

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

Removing the .DS_Store file eliminated the warning.