Add --linux-compat build option for Ubuntu 20.04 binaries

Build release binaries inside an Ubuntu 20.04 Docker container
to produce executables with lower GLIBC requirements, compatible
with older Linux distributions.

- Add Dockerfile.compat (Ubuntu 20.04 base, full depends rebuild)
- Add .dockerignore to exclude host build artifacts from context
- Add --linux-compat flag to build.sh with Docker build/extract/package
- Strip binaries inside container to avoid root ownership issues

.dockerignore

@@ -0,0 +1,27 @@
+.git
+release
+depends/built
+depends/work
+depends/x86_64-unknown-linux-gnu
+depends/x86_64-w64-mingw32
+src/RandomX/build
+src/*.o
+src/*.a
+src/*.la
+src/*.lo
+src/.libs
+src/.deps
+src/univalue/.libs
+src/univalue/.deps
+src/cc/*.o
+src/cc/*.a
+src/dragonxd
+src/dragonx-cli
+src/dragonx-tx
+src/dragonxd.exe
+src/dragonx-cli.exe
+src/dragonx-tx.exe
+sapling-output.params
+sapling-spend.params
+config.status
+config.log