Add Dockerfile for standalone use

Example usage:
	docker build -t geyser -f standalone.Dockerfile https://github.com/GeyserMC/Geyser.git
	docker run -p 19132:19132/udp -v `pwd`/geyser:/var/lib/geyser geyser

.dockerignore

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+.dockerignore
+*Dockerfile

standalone.Dockerfile

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+FROM maven:3.6-jdk-11 as build
+
+COPY . .
+
+RUN git submodule update --init --recursive&&\
+	mvn clean install
+
+FROM adoptopenjdk/openjdk11:alpine-jre
+
+COPY --from=build ./bootstrap/standalone/target /opt/Geyser
+
+ARG UID=1000
+ARG GID=1000
+RUN adduser --system --shell /bin/false -u $UID -g $GID --home /opt/Geyser geyser
+
+RUN mkdir -v /var/lib/geyser && chown -v -R ${UID}:0 /var/lib/geyser
+VOLUME /var/lib/geyser
+
+USER geyser
+WORKDIR /var/lib/geyser
+EXPOSE 19132/udp
+CMD ["java", "-jar", "/opt/Geyser/Geyser.jar"]