Docker: split Dockerfile into multi-stage build to shrink runtime image

[csparker247]

Background

The Docker image added in #12 is a single-stage build. The final image therefore ships the entire build toolchain — build-essential, cmake, ninja, git, the -dev packages for VTK/OpenCV/Boost/TIFF/spdlog/Eigen, plus everything needed to compile ITK from source — even though none of that is needed at runtime. (The ITK and toolkit source/build trees are already cleaned up; this is about the apt build dependencies and headers.)

Proposal

Split the Dockerfile into a builder stage and a runtime stage:

# ---- builder: compilers, headers, sources ----
FROM ${BASE_IMAGE} AS builder
RUN apt install ... build-essential cmake ninja git lib*-dev
RUN git clone ITK && cmake --install ... /usr/local
COPY . /src
RUN cmake --install ... /usr/local

# ---- runtime: runtime libs + installed artifacts only ----
FROM ${BASE_IMAGE} AS runtime
RUN apt install ... imagemagick libvtk9.1 libopencv-core4.5d ...   # runtime libs, no -dev, no compilers
COPY --from=builder /usr/local /usr/local
RUN ldconfig
CMD ["rt_register", "--help"]

Expected gain

Dropping the compiler toolchain, ITK source build, and -dev packages should cut the final image by several hundred MB (likely >1 GB).

Work / risks

• Enumerate the runtime package names (e.g. libvtk9.1, libopencv-imgproc4.5d, libboost-program-options1.83.0, libtiff6, libspdlog1.x) instead of the -dev metapackages. These names can differ across Ubuntu releases and occasionally between amd64/arm64.
• Requires a real build + run test on both amd64 and arm64 to confirm nothing is missing at load time (ldd on the rt_* binaries is a good check).
• Orthogonal to the build-cache work already in Add Docker image build and CI workflow #12 — this is purely about final image size.

Deferred from #12 review as a follow-up so it can get proper cross-arch verification.