Configure build+test matrix for GitLab CI:

* Disable parallel tests for address sanitizer
* Improve caching
* Prefer Ninja builds because they are faster

bin/ci/README.md

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+In this directory are two scripts, `build.sh` and `test.sh` used for building
+and testing rippled.
+
+(For now, they assume Bash and Linux. Once I get Windows containers for
+testing, I'll try them there, but if Bash is not available, then they will
+soon be joined by PowerShell scripts `build.ps` and `test.ps`.)
+
+We don't want these scripts to require arcane invocations that can only be
+pieced together from within a CI configuration. We want something that humans
+can easily invoke, read, and understand, for when we eventually have to test
+and debug them interactively. That means:
+
+(1) They should work with no arguments.
+(2) They should document their arguments.
+(3) They should expand short arguments into long arguments.
+
+While we want to provide options for common use cases, we don't need to offer
+the kitchen sink. We can rightfully expect users with esoteric, complicated
+needs to write their own scripts.
+
+To make argument-handling easy for us, the implementers, we can just take all
+arguments from environment variables. They have the nice advantage that every
+command-line uses named arguments. For the benefit of us and our users, we
+document those variables at the top of each script.