Added Dockerfile to build Docker image for backend.

Dockerfile

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+## BACKEND ##
+
+# Base image
+FROM debian:12-slim
+
+# Metadata
+LABEL maintainer="kshitijka"
+LABEL version=1.0
+LABEL description="Skycrate is a web based file management system that uses Hadoop as filesystem."
+
+# Update & upgrade & install & rm
+RUN apt-get update && apt-get upgrade -y && \
+    apt-get install -y openjdk-17-jdk && \
+    rm -rf /var/lib/apt/lists/* 
+
+# Create non-root user
+RUN useradd -s /bin/bash skycrateBack
+
+# Create work dir
+RUN mkdir /app
+RUN chown -R skycrateBack:skycrateBack /app
+COPY ./Skycrate/target/ /app
+WORKDIR /app
+
+# Switch user
+USER skycrateBack
+
+# EXPOSE 8080
+
+CMD ["java", "-jar", "/app/skycrateBackend-0.0.1-SNAPSHOT.jar"]