CI: move docker build machinery to the main repo

more details: https://github.com/SoftEtherVPN/SoftetherVPN-docker/issues/4

Dockerfile

@@ -0,0 +1,51 @@
+FROM alpine AS builder
+RUN mkdir /usr/local/src && apk add binutils --no-cache\
+        linux-headers \
+	build-base \
+        readline-dev \
+        openssl-dev \
+        ncurses-dev \
+        git \
+        cmake \
+        zlib-dev \
+        libsodium-dev \
+        gnu-libiconv 
+
+ENV LD_PRELOAD=/usr/lib/preloadable_libiconv.so
+ADD ./ /usr/local/src/SoftEtherVPN/
+WORKDIR /usr/local/src
+ENV USE_MUSL=YES
+ENV CMAKE_FLAGS="-DSE_PIDDIR=/run/softether -DSE_LOGDIR=/var/log/softether -DSE_DBDIR=/var/lib/softether"
+RUN cd SoftEtherVPN &&\
+        ./configure &&\
+	make -j $(getconf _NPROCESSORS_ONLN) -C build
+
+FROM alpine AS base
+RUN apk add --no-cache readline \
+        openssl \
+        libsodium \
+        gnu-libiconv \
+        iptables
+ENV LD_PRELOAD=/usr/lib/preloadable_libiconv.so
+WORKDIR /usr/local/bin
+VOLUME /var/log/softether
+VOLUME /var/lib/softether
+VOLUME /run/softether
+COPY --from=builder /usr/local/src/SoftEtherVPN/build/vpncmd /usr/local/src/SoftEtherVPN/build/hamcore.se2 ./
+COPY --from=builder /usr/local/src/SoftEtherVPN/build/libcedar.so /usr/local/src/SoftEtherVPN/build/libmayaqua.so /usr/local/lib/
+
+
+FROM base AS vpnserver
+COPY --from=builder /usr/local/src/SoftEtherVPN/build/vpnserver ./
+EXPOSE 443/tcp 992/tcp 1194/tcp 1194/udp 5555/tcp 500/udp 4500/udp
+CMD ["/usr/local/bin/vpnserver", "execsvc"]
+
+
+FROM base AS vpnclient
+COPY --from=builder /usr/local/src/SoftEtherVPN/build/vpnclient ./
+CMD ["/usr/local/bin/vpnclient", "execsvc"]
+
+
+FROM base AS vpnbridge
+COPY --from=builder /usr/local/src/SoftEtherVPN/build/vpnbridge ./
+CMD ["/usr/local/bin/vpnbridge", "execsvc"]