Ubuntu 26.04: Fix OCCT dependencies to avoid VTK/Java/GCC-16 bloat

The libocct-*-dev packages in Ubuntu 26.04 have a hard dependency on
libocct-ivtk-dev, which pulls in the full VTK stack, Java (OpenJDK 25),
MPI, the GCC-16 compiler suite, Boost dev, GDAL, HDF5, media codecs,
and ~2GB of other transitive bloat.

To work around this Ubuntu 26.04 packaging issue:
1. Install the OCCT runtime packages (libocct-*-7.9) via apt normally -
   they have no VTK dependency.
2. Extract the dev package files (headers, cmake configs, .so symlinks)
   using dpkg-deb -x, bypassing apt dependency resolution entirely.
3. Also extract libocct-ivtk-7.9 (the 112K runtime .so) so that cmake's
   OpenCASCADEVisualizationTargets.cmake can find libTKIVtk.so.7.9.
4. Remove cargo registry/git cache after cargo install to save space.

Verified: cmake find_package(OpenCASCADE) succeeds and a test program
using BRep/STEP headers compiles successfully, with no VTK or Java
installed in the image.

Agent-Logs-Url: https://github.com/LibrePCB/docker-librepcb-dev/sessions/13556661-f78f-4ed6-8f93-71071bd01171

Co-authored-by: ubruhin <5374821+ubruhin@users.noreply.github.com>

Author: Copilot

ubuntu-26.04/Dockerfile

@@ -27,7 +27,18 @@ RUN apt-get update -q && apt-get -y -q install --no-install-recommends \
     libgtest-dev \
     libmuparser-dev \
     libmuparser2v5 \
-    libocct-*-dev \
+    # The libocct-*-dev packages have an unnecessary hard dependency on
+    # libocct-ivtk-dev, which pulls in VTK, Java, MPI, the GCC-16 compiler
+    # suite, and ~2GB of other bloat. To work around this Ubuntu 26.04
+    # packaging issue, we install the OCCT runtime packages here via apt
+    # and extract the dev package files (headers, cmake configs, .so symlinks)
+    # separately below using dpkg-deb, bypassing the broken dependency.
+    libocct-data-exchange-7.9 \
+    libocct-foundation-7.9 \
+    libocct-modeling-algorithms-7.9 \
+    libocct-modeling-data-7.9 \
+    libocct-ocaf-7.9 \
+    libocct-visualization-7.9 \
     libpolyclipping-dev \
     libpolyclipping22 \
     libqt6opengl6-dev \
@@ -52,6 +63,22 @@ RUN apt-get update -q && apt-get -y -q install --no-install-recommends \
     xvfb \
     zlib1g \
     zlib1g-dev \
+  # Extract OCCT dev package files (headers, cmake configs, .so symlinks) without
+  # going through apt install (which would pull in the unwanted ivtk dependency).
+  # Also extract libocct-ivtk-7.9 (112K .so) so cmake's
+  # OpenCASCADEVisualizationTargets.cmake can find libTKIVtk.so.7.9.
+  && cd /tmp \
+  && apt-get download \
+       libocct-data-exchange-dev \
+       libocct-draw-dev \
+       libocct-foundation-dev \
+       libocct-modeling-algorithms-dev \
+       libocct-modeling-data-dev \
+       libocct-ocaf-dev \
+       libocct-visualization-dev \
+       libocct-ivtk-7.9 \
+  && for f in /tmp/libocct-*.deb; do dpkg-deb -x "$f" /; done \
+  && rm -f /tmp/libocct-*.deb \
   && rm -rf /var/lib/apt/lists/*
 
 # Install Rust
@@ -63,6 +90,7 @@ ENV RUSTUP_HOME="/.rustup" \
 RUN curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs \
     | sh -s -- -y --default-toolchain $RUST_VERSION --profile minimal --no-modify-path \
   && cargo install cargo-llvm-cov \
+  && rm -rf $CARGO_HOME/registry $CARGO_HOME/git \
   && chmod -R 777 $RUSTUP_HOME \
   && chmod -R 777 $CARGO_HOME