Update docker build target

[retgits]

What kind of change does this PR introduce? (check one with "x")

[X] Bugfix
[] Feature
[] Code style update (formatting, local variables)
[] Refactoring (no functional changes, no api changes)
[] Other... Please describe:

Fixes: # https://github.com/TIBCOSoftware/flogo/issues/293

What is the current behavior? The current build command for docker flogo build -docker produces a dockerfile with an error. The ADD command adds the contents of the linux directory directly in the root and not in a subdirectory. This means the CMD command will not find the right executable. On linux systems it won't create the linux sub directory at all, so the command would fail there too

What is the new behavior?

• The ADD command has been replaced with COPY to follow docker best practices
• CGO will be disabled for these builds, as enabling CGO on Linux machines will create dynamically linked executables