feat: use non root user for Dockerfile

fixes #51

Dockerfile

@@ -1,7 +1,20 @@
-FROM ubuntu:20.04
+FROM ubuntu:22.04
 RUN apt-get -y update && apt install -y ca-certificates
+
 WORKDIR /app
+
 COPY calert.bin .
 COPY static/ /app/static/
 COPY config.sample.toml config.toml
+
+ARG CALERT_GID="999"
+ARG CALERT_UID="999"
+
+RUN addgroup --system --gid $CALERT_GID calert && \
+    adduser --uid $CALERT_UID --system --ingroup calert calert && \
+    chown -R calert:calert /app
+
+USER calert
+EXPOSE 6000
+
 ENTRYPOINT [ "./calert.bin" ]