[chore](trx-rs): use nested SDK image path trx-rs/sdk

Match the image name that was pushed to the registry
(git.haxx.space/sjg/trx-rs/sdk) across the workflow, devcontainer and
README.

Assisted-By: Claude Code (claude-opus-4)
Claude-Session: https://claude.ai/code/session_01NFpGtGTWUEYXLwZeZs2RAV
Signed-off-by: Stan Grams <sjg@haxx.space>

container/README.md

@@ -19,9 +19,9 @@ and `clippy` are identical everywhere — no "works on my machine".
 
 ```bash
 # from the repo root
-podman build -t git.haxx.space/sjg/trx-rs-sdk:latest container
+podman build -t git.haxx.space/sjg/trx-rs/sdk:latest container
 podman login git.haxx.space
-podman push git.haxx.space/sjg/trx-rs-sdk:latest
+podman push git.haxx.space/sjg/trx-rs/sdk:latest
 ```
 
 Tag with the Rust version too (e.g. `:1.97.1`) if you want reproducible pins.
@@ -36,7 +36,7 @@ Reproducible one-off build, no local toolchain needed:
 
 ```bash
 podman run --rm -it -v "$PWD":/work -w /work \
-  git.haxx.space/sjg/trx-rs-sdk:latest \
+  git.haxx.space/sjg/trx-rs/sdk:latest \
   cargo build --release
 ```