fix(logging): Replace log-level parameter by GATUS_LOG_LEVEL env var (#895)

* fix(logging): Replace log-level parameter by GATUS_LOG_LEVEL env var

* Improve log message if GATUS_LOG_LEVEL isn't set

Dockerfile

@@ -15,6 +15,8 @@ FROM scratch
 COPY --from=builder /app/gatus .
 COPY --from=builder /app/config.yaml ./config/config.yaml
 COPY --from=builder /etc/ssl/certs/ca-certificates.crt /etc/ssl/certs/ca-certificates.crt
-ENV PORT=8080
+ENV GATUS_CONFIG_PATH=""
+ENV GATUS_LOG_LEVEL="INFO"
+ENV PORT="8080"
 EXPOSE ${PORT}
 ENTRYPOINT ["/gatus"]