README.md: build image with older spack, spack-packages, spack-config (…

…#174)

* Also added Tommy's feedback on describing the difference between the two build examples.

containers/README.md

@@ -1,9 +1,19 @@
-### Building Docker containers for local testing
+## Requirements
 
-To build:
+* Build a container using the same OS, `spack`, `spack-packages` and `spack-config` versions as an official release of a product built via Spack. e.g. Rocky Linux 8.10, `spack` `v0.20`, `spack-packages` `2024.03.22`, `spack-config` `2024.03.22`.
+
+* `spack` (branch-based versioning) and `spack-config` (directory-based versioning) are versioned based on the Spack version. Since `spack-packages` is not versioned, we have to use tags, e.g.  `2024.03.22`, to extract an older version that was created to be used with an older version of Spack.
+
+## How to build Docker images
+
+To build an image with defaults:
 
     docker build -f Dockerfile.base-spack -t <name>:<version> --target dev --no-cache --progress=plain .
 
+To build an image with specific versions of `spack`, `spack-packages` and `spack-config`:
+
+    docker build -f Dockerfile.base-spack -t spack-dev:v0.20 --target dev --build-arg SPACK_VERSION=v0.20 --build-arg SPACK_PACKAGES_REPO_VERSION=2024.03.22 --build-arg SPACK_CONFIG_REPO_VERSION=2024.03.22 --no-cache --progress=plain .
+
 To run:
 
     docker run -it --rm <name>:<version>