docker: fix ARG scoping, remove cache-defeating cleanup, quiet tar

[beriberikix]

Summary

Fix three categories of Docker build warnings identified in GHA annotations and local build output.

Changes

W1 — Remove cache-defeating apt-get clean / rm -rf /var/lib/apt/lists/* (all 3 Dockerfiles)

The --mount=type=cache mounts on /var/cache/apt and /var/lib/apt/lists keep those directories in BuildKit's persistent cache — they are never written to the image layer. Calling apt-get clean and rm -rf /var/lib/apt/lists/* at the end of those same RUN steps wiped the BuildKit cache on every build, forcing a full apt list re-download on the next run. Removing these lines means the cache actually persists between builds as intended.

apt-get autoremove is kept — it removes unneeded packages from the image layer, which is correct.

W2 — Re-declare ARGs with defaults after FROM (zephyr/Dockerfile, zephyr-posix/Dockerfile)

Docker ARGs declared before FROM go out of scope after FROM. Both child Dockerfiles re-declared ARG ZEPHYR_SDK_INSTALL_DIR with no default, so the variable resolved to an empty string post-FROM, making ${ZEPHYR_SDK_INSTALL_DIR}/setup.sh resolve to /setup.sh. The BuildKit 1.7 linter warns about undefined variables used in commands.

Fix: re-declare all consumed ARGs after FROM with explicit defaults (ZEPHYR_SDK_VERSION=1.0.0, ZEPHYR_SDK_INSTALL_DIR=/opt/toolchains/zephyr-sdk-${ZEPHYR_SDK_VERSION}, and ZEPHYR_SDK_TOOLCHAINS in zephyr/Dockerfile). CI-supplied --build-arg values override these defaults as before.

W3 — tar -xvf to tar -xf (zephyr-base/Dockerfile)

The -v (verbose) flag printed every filename in the SDK archive to stdout, producing thousands of lines of build output that obscured real warnings. Quieted to -xf.

Testing

All three images built successfully locally:

• zephyr:base-1.0.0SDK (arm64)
• zephyr:arm-1.0.0SDK (arm64)
• zephyr:posix-1.0.0SDK (amd64 — pre-existing platform constraint due to gcc-multilib)

Build output is clean: no verbose tar listing, no BuildKit ARG-undefined warnings.