Create an API for retrieving the path and absolute path of a image

[rcjsuen]

Need to add two new API functions to the Workdir class.

• getPath() will return the path has been set in the WORKDIR instruction
• getAbsolutePath() will try to return an absolute path based on the relative path defined in the WORKDIR instruction and any prior WORKDIR instruction, or just the absolute path if that's what was set in the WORKDIR instruction

Support for Windows containers will not be addressed at this time. getPath() may be used to construct the absolute path manually if this is desired.