Feature/podman

[carroarmato0]

The build system previously had Docker hardcoded as the only container runtime. This PR adds support for Podman as a drop-in alternative, which is useful on systems where Docker is not available or not preferred (e.g. rootless Linux setups where Podman is the default).

makefile.toolchain:

• Detects available container runtime at build time: Docker is preferred when both are installed, Podman is used as a fallback when Docker is absent
• Allows explicitly overriding the runtime via CONTAINER_RUNTIME=podman on the make command line
• Tracks the last-used runtime in workspace/.container_runtime and automatically cleans workspace artifacts when switching
  between runtimes

Automatic cleanup is needed as Docker containers run as root, so compiled objects and cloned repos (e.g. other/NextCommander) end up root-owned on the host. Rootless Podman cannot write into those directories. When a runtime switch is detected, the cleanup runs inside the previous container using find -depth -user root -delete, which removes only what that runtime created while leaving host-owned source files untouched. If the previous runtime is no longer installed, an actionable manual cleanup command is printed and the build aborts rather than producing a half-broken state.

workspace/tg5040/btmanager/Makefile:

• Added missing clean target (pre-existing gap, caught during testing when the cleanup path exercised it for the first time)

Examples:

# uses docker if available, podman otherwise
make PLATFORM=tg5040 build

# explicitly use podman
make PLATFORM=tg5040 build CONTAINER_RUNTIME=podman

# switching runtimes is handled automatically - no manual clean needed
make PLATFORM=tg5040 build                           # builds with docker, records runtime
make PLATFORM=tg5040 build CONTAINER_RUNTIME=podman  # detects switch, cleans, rebuilds

Build tests:

Docker (29.4.1) — successful build:
make build -f makefile.toolchain PLATFORM=tg5040 COMPILE_CORES=
docker run --rm -v .../workspace:/root/workspace ...
...
echo "skipping core build"
skipping core build
# ----------------------------------------------------
Artifacts owned by root on the host (expected Docker behaviour).

Podman (5.8.2) — successful build after automatic cleanup:
make build -f makefile.toolchain PLATFORM=tg5040 COMPILE_CORES=
Container runtime changed (docker -> podman), cleaning workspace build artifacts...
podman run --rm -v .../workspace:/root/workspace ...
...
echo "skipping core build"
skipping core build
# ----------------------------------------------------
Artifacts owned by the host user after the build, confirming rootless Podman mapped ownership correctly:
-rwxr-xr-x 1 carroarmato0 carroarmato0 557576 Apr 30 09:04 workspace/all/minarch/build/tg5040/minarch.elf
-rwxr-xr-x 1 carroarmato0 carroarmato0 149728 Apr 30 09:04 workspace/all/nextui/build/tg5040/nextui.elf