Change "-Djib.dockerDir" in maven plugin for consistency

[TadCordle]

In gradle, the configuration option is targetDir, and the parameter is --targetDir.

In maven, the configuration option is targetDir, and the parameter is -Djib.dockerDir. We should make it more consistent (something like -Djib.targetDir, or maybe even just -DtargetDir).

[chanseokoh]

Personally I vote for jib.targetDir. -DtargetDir sounds too general that may easily conflict with others.

[coollog]

I believe the issue with Gradle is that it doesn't allow for . in the property name. It might have to be jibTargetDir

[TadCordle]

Change it to jibTargetDir for both?

[coollog]

Sounds good!