Add dpkg/rpm building capability:

* docker container definitions for package building
* cmake targets for building packages
* initial gitlab CI + artifactory integration

Builds/containers/README.md

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+
+# rippled Packaging and Containers
+
+This folder contains docker container definitions and configuration
+files to support building rpm and deb packages of rippled. The container
+definitions include some additional software/packages that are used
+for general build/test CI workflows of rippled but are not explicitly
+needed for the package building workflow.
+
+## CMake Targets
+
+If you have docker installed on your local system, then the main 
+CMake file will enable several targets related to building packages:
+`rpm_container`, `rpm`, `dpkg_container`, and `dpkg`. The package targets
+depend on the container targets and will trigger a build of those first.
+The container builds can take several dozen minutes to complete (depending
+on hardware specs), so quick build cycles are not possible currently. As
+such, these targets are often best suited to CI/automated build systems.
+
+The package build can be invoked like any other cmake target from the 
+rippled root folder:
+```
+mkdir -p build/pkg && cd build/pkg
+cmake -Dpackages_only=ON ../..
+cmake --build . --target rpm
+```
+Upon successful completion, the generated package files will be in 
+the `build/pkg/packages` directory. For deb packages, simply replace
+`rpm` with `dpkg` in the build command above.
+
+