Revert Dockerfile changes (#581)

Dockerfile

@@ -1,9 +1,7 @@
 FROM alpine:3.12
-RUN addgroup -g 1000 -S appgroup && adduser -u 1000 -S appuser -G appgroup
 RUN apk --no-cache add ca-certificates git rpm
 COPY trivy /usr/local/bin/trivy
 COPY contrib/gitlab.tpl contrib/gitlab.tpl
 COPY contrib/junit.tpl contrib/junit.tpl
 COPY contrib/sarif.tpl contrib/sarif.tpl
-USER appuser
 ENTRYPOINT ["trivy"]

README.md

@@ -1955,19 +1955,6 @@ Try again with `--reset` option:
 ```
 $ trivy image --reset
 ```
-
-### Error: FATAL	unable to initialize the cache: failed to create cache dir: mkdir /root/.cache: permission denied
-
-```
-$ docker run --rm -v $PWD:/root/.cache/  aquasec/trivy:latest --cache-dir /root/.cache image centos:7
-2020-07-29T15:02:54.435Z	FATAL	unable to initialize the cache: failed to create cache dir: mkdir /root/.cache: permission denied
-```
-
-Try:
-
-```
-$ docker run --rm -v $PWD:/tmp/.cache/  aquasec/trivy:latest --cache-dir /tmp/.cache image centos:7
-```
 ---
 
 # Credits