is there any way to preserve the user image when it is a docker base image?

well… there’s no way to do it the way you’d like it but you can always use a workaround with multistage builds:

FROM solr:alpine as build

RUN whoami

FROM build as prepare_dependencies
USER root
RUN echo 'I am root' > /the_root_important_message

FROM build

COPY --from=prepare_dependencies /the_root_important_message /the_root_vital_message

RUN echo "now I'm $( whoami )"
RUN cat /the_root_vital_message
CMD echo 'this is a pain'

Somehow I’m fairly sure this is not what you’re looking for…

Well, since this topic is pretty fun, I decided to attempt a different approach:

FROM solr:alpine as build

RUN whoami

FROM build as prepare_dependencies
USER root
RUN apk --no-cache --update add sudo \
    && echo 'ALL ALL=(ALL) NOPASSWD: ALL' > /etc/sudoers

RUN echo "this is secret" > /secretfile
RUN chmod 400 /secretfile

FROM build

COPY --from=prepare_dependencies /usr/bin/sudo /usr/bin/sudo
COPY --from=prepare_dependencies /usr/lib/sudo /usr/lib/sudo
COPY --from=prepare_dependencies /etc/sudoers /etc/sudoers
COPY --from=prepare_dependencies /secretfile /secretfile

RUN sudo -l
RUN sudo cat /secretfile

RUN echo "now I'm $( whoami )"

RUN echo "cleanup" \
    && sudo rm -rf \
        /etc/sudoers /secretfile \
        /usr/lib/sudo /usr/bin/sudo

CMD echo 'this is a pain'