Share ccache volume across checkouts, cap at 2G

The ccache named volume had no explicit name, so Docker Compose
namespaced it under the per-checkout project name (derived from
UNIQUE_ID), giving each checkout its own cache even though
docker/README.md already documented them as shared. Give the volume
a fixed name so all checkouts attach the same one.

Measured cache size after a full build (IfcParse+IfcGeom+IfcConvert+
wrapper, one Python version) is ~300MB, only ~5% of the previous 5G
cap. Shrink CCACHE_MAXSIZE to 2G, which comfortably covers the shared
baseline plus per-branch deltas from several diverging checkouts.

Generated with the assistance of an AI coding tool.

(cherry picked from commit b1470223d3)

docker/Dockerfile

@@ -28,8 +28,11 @@ RUN git config --system --add safe.directory '*'
 # volume mount point at runtime - anything `ccache -M` writes to a config
 # file under it during this build gets shadowed once the real volume is
 # mounted, so the size cap only actually takes effect via the env var.
+# 2G is generous: a full build (IfcParse+IfcGeom+IfcConvert+wrapper, one
+# Python version) measures ~300MB, and the volume is now shared across all
+# checkouts (see compose.yaml), so this covers several diverging branches.
 ENV CCACHE_DIR=/ccache
-ENV CCACHE_MAXSIZE=5G
+ENV CCACHE_MAXSIZE=2G
 ENV PATH="/usr/lib/ccache:$PATH"
 
 # Non-root user matching the host UID/GID that bind-mounts the repo (default

docker/compose.yaml

@@ -20,3 +20,4 @@ services:
 
 volumes:
   ccache:
+    name: ifcopenshell-ccache-shared