add CI files

.docker/Dockerfile

@@ -0,0 +1,26 @@
+FROM gentoo/stage3:latest
+
+COPY ./. /
+
+RUN getuto
+
+RUN emerge-webrsync
+
+RUN emerge \
+    =dev-lang/rust-bin-9999 \
+    llvm-core/clang \
+    llvm-core/lld \
+    dev-vcs/git \
+    sys-process/parallel
+
+RUN git clone https://jturnerusa.dev/cgit/ebuilds/ /var/db/repos/spawns
+
+COPY etc/portage/repos.conf /etc/portage/
+
+RUN mkdir -p /var/cache/distfiles/git3-src && chown portage:portage /var/cache/distfiles/git3-src
+
+RUN emerge =dev-build/meson-9999
+
+WORKDIR /workspace
+
+ENTRYPOINT /bin/bash