Fix warning that results when providing new --chown COPY flag

[romank8k]

This fixes logging an invalid warning that results when providing the new --chown flag to the COPY command in a custom Dockerfile.

[codecov]

Codecov Report

Merging #884 into master will increase coverage by 0.13%.
The diff coverage is 58.33%.

@@             Coverage Diff              @@
##             master     #884      +/-   ##
============================================
+ Coverage     50.84%   50.98%   +0.13%     
- Complexity     1208     1213       +5     
============================================
  Files           140      140              
  Lines          7123     7129       +6     
  Branches        953      956       +3     
============================================
+ Hits           3622     3635      +13     
+ Misses         3189     3178      -11     
- Partials        312      316       +4

Impacted Files	Coverage Δ	Complexity Δ
...8/maven/docker/assembly/DockerAssemblyManager.java	17.62% <58.33%> (+5.01%)	11 <0> (+5)	⬆️

[romank8k]

Forgot to mention, the --chown option is part of the release notes from 2017-09-26: https://docs.docker.com/release-notes/docker-ce/

[rhuss]

Thanks ! I hope I can review the PR soon ...

[rhuss]

Thanks ! I adapted a bit and the fix will be available in the version to be released on this weekend.