build: stamp container builds with the real version instead of "-unk"

.dockerignore excludes .git, so util/genbuild.sh finds no repository inside
the container and emits "// No build information available", which
clientversion.cpp renders as the "-unk" suffix. Every binary produced by
./build.sh --linux-compat therefore self-reports "v1.2.0-unk" and cannot be
traced to a commit -- including release artifacts, since this build path is
part of the v1.2.0 tag.

Pre-generating src/obj/build.h does not survive (genbuild rewrites it when the
content differs), and simply un-ignoring .git does not help a linked worktree,
whose .git is a file pointing outside the build context.

So build.sh computes the version on the host, mirroring genbuild.sh rule for
rule -- the nearest tag only when HEAD is that tag and the tree is clean,
otherwise v<VERSION>-<short sha> with a -dirty suffix -- and passes it through
a BUILD_DESC build-arg that Dockerfile.compat exports as DRAGONX_BUILD_DESC.
genbuild.sh honours that variable when set and is otherwise untouched; with
git metadata present it emits a byte-identical build.h.

Every added git call is guarded with || true because build.sh runs under
set -eu -o pipefail: a source tarball, a host without git, or a branch whose
only reachable tags are lightweight (v1.0.1-v1.0.3 are lightweight; v1.1.0 is
the first annotated one) would otherwise abort the build with no diagnostic.
Those cases now degrade to the previous "-unk" behaviour with a warning.

A direct "docker build -f Dockerfile.compat" passes no BUILD_DESC and still
produces -unk; the Dockerfile now says so loudly rather than silently.

Dockerfile.compat

@@ -27,6 +27,17 @@ RUN rm -rf /build/depends/built /build/depends/work \
     && rm -rf /build/src/cc/*.o /build/src/cc/*.a \
     && rm -f /build/config.status /build/config.log
 
+# The build context excludes .git (see .dockerignore), so genbuild.sh cannot derive
+# a version and would stamp the binaries "-unk". build.sh computes the real one
+# on the host and passes it in here.
+ARG BUILD_DESC=
+ENV DRAGONX_BUILD_DESC=${BUILD_DESC}
+
+RUN if [ -z "$DRAGONX_BUILD_DESC" ]; then \
+      echo "WARNING: no BUILD_DESC build-arg -- binaries will be stamped -unk." >&2; \
+      echo "         Prefer ./build.sh --linux-compat, or pass --build-arg BUILD_DESC=..." >&2; \
+    fi
+
 RUN cd /build && ./util/build.sh --disable-tests -j$(nproc)
 
 # Strip binaries inside the container so extracted files are already small