Update README for building the CI docker image (#26757)

Update the README for building the CI docker image to reflect the
current PATHS

[Reviewed by @riftEmber]

util/packaging/docker/github-ci/Readme.md

@@ -1,11 +1,23 @@
+# Github CI Docker Container
+
+This directory contains the `Dockerfile` for the Chapel GitHub CI container.
+Running the container will replicate the environment used by the CI. After
+launching the container, you can run parts of the CI manually. See
+`$CHPL_HOME/.github/workflows/CI.yml` for the steps the CI runs.
+
+## Running the container
+
 `docker` or `podman` should both work:
 
-```
-docker build -f util/dockerfiles/github-ci/Dockerfile -t chapel-lang/chapel-github-ci:latest
+These commands should be run from this directory (the one containing the
+`Dockerfile`).
+
+```bash
+docker build . -t chapel-lang/chapel-github-ci:latest
 # -w working directory
 # -v volume mount
 # -it interactive with tty
 # --rm remove container at exit
-docker run -w="$(pwd)" -v"$(pwd)":"$(pwd)":z -it --rm chapel-lang/chapel-github-ci:latest
+docker run -w="$CHPL_HOME" -v"$CHPL_HOME":"$CHPL_HOME":z -it --rm chapel-lang/chapel-github-ci:latest
 # NOTE: this mounts `build` and any `third-party/**/{build,install}` into the container, so be aware
 ```