Add docker files for local testing.

* Use docker to locally run tests on multiple perl versions
    * perlbrew used to install multiple perls
    * perl environments bootstrapped when image built
* Based on travis configuration
* Adds docker folder to MANIFEST.SKIP
* Running image runs all tests.
    * If exit with no error, then all tests pass.
* ./build-docker.sh builds stow testing image.
* ./test-docker.sh runs stow testing image.
    * Assumes built with options in build-docker.sh
* *-docker.sh scripts added to MANIFEST.SKIP

MANIFEST.SKIP

@@ -83,3 +83,5 @@
 # Avoid test files
 tmp-testing-trees
 .travis.yml
+^docker/
+^[a-zA-Z]*-docker.sh