Allow commandline arguments to be passed to docker binary

[jakobnordztrom]

Backwards compatibility maintained so that you can still set it as a string, e.g dockerBinary="mydockerbin".
To pass options use an array, e.g dockerBinary=["docker","-H", "1.2.3.4:2375" ]

[codecov-io]

Codecov Report

Merging #152 into master will increase coverage by 8.4%.
The diff coverage is 64.28%.

@@             Coverage Diff             @@
##             master     #152     +/-   ##
===========================================
+ Coverage     59.17%   67.58%   +8.4%     
- Complexity       82       92     +10     
===========================================
  Files             9       10      +1     
  Lines           316      327     +11     
  Branches         26       26             
===========================================
+ Hits            187      221     +34     
+ Misses          116       88     -28     
- Partials         13       18      +5

Impacted Files	Coverage Δ	Complexity Δ
...e/plugins/docker/client/CommandExecutorImpl.groovy	0% <0%> (ø)	1 <1> (?)
...nsmode/gradle/plugins/docker/DockerTaskBase.groovy	90.32% <100%> (+0.66%)	15 <2> (+2)	⬆️
...ransmode/gradle/plugins/docker/DockerPlugin.groovy	62.71% <100%> (ø)	8 <0> (ø)	⬇️
...le/plugins/docker/client/NativeDockerClient.groovy	80.95% <78.94%> (+75.82%)	8 <3> (+7)	⬆️

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