WIP: Enable gem package listing

[abhaykatheria]

This commit adds changes to snippets.yml and base.yml to
collect the metadata of gem modules. Scripts are added
to base.yml to list the version, license, proj_url.

This PR is work towards #609.

Signed-off-by: abhaykatheria [email protected]

[abhaykatheria]

I have tested these changes on rails:latest image from docker hub. Here the result from the last layer

	Layer: 07032e5274:
		info: Instruction Line: RUN gem install rails --version "$RAILS_VERSION"
		info: Layer created by commands: /bin/sh -c gem install rails --version "$RAILS_VERSION"
		info: Retrieved by invoking listing in command_lib/base.yml
licenses:
	in container:
	pkgs=`gem list 2> /dev/null | awk '{print $1}'`
	for pkg in $pkgs;do gem info $pkg 2>/dev/null |grep -n 'License:'| awk '{print $3}'; done

proj_urls:
	in container:
	pkgs=`gem list 2> /dev/null | awk '{print $1}'`
	for pkg in $pkgs;do gem info $pkg 2>/dev/null |grep -n 'Homepage:'| awk '{print $3}'; done

versions:
	in container:
	gem list 2> /dev/null | awk '{print $2}' | awk -F'[()]' '{print $2}'

names:
	in container:
	gem list 2> /dev/null | awk '{print $1}'



	Invoking commands from command_lib/base.yml:
		warning: No listing for key srcs. Consider adding this listing to command_lib/base.yml.
No listing for key copyrights. Consider adding this listing to command_lib/base.yml.

	File licenses found in Layer:  None
	Packages found in Layer:  bigdecimal-default:, bundler-1.13.7, did_you_mean-1.0.0, io-console-default:, json-default:, minitest-5.8.5, net-telnet-0.1.1, power_assert-0.2.6, psych-default:, rake-10.4.2, rdoc-default:, rubygems-update-2.6.8, test-unit-3.1.5
	Licenses found in Layer:  None
------------------------------------------------

[rnjudge]

Thanks for opening this @abhaykatheria! I took this commit and modified some of it to display the proper package version, license and proj_url information. I opened a new PR here: #663

Thanks again for getting the ball rolling on this!

[nishakm]

@abhaykatheria @rnjudge closing this as I have merged the other PR.