Add MAD-X spack package

• MAD-X accelerator simulation.
• https://github.com/MethodicalAcceleratorDesign/MAD-X
• Started with a copy of ACTS' package

containers/builder/spack.yaml

 spack:
   specs:
+    - madx@5.06.1
     - cmake@3.18.1
     - fmt@7.1.2 cxxstd=17
     - spdlog@1.5.0