Cannot COPY from previous stage in DockerFIle - docker

Question

This looks like a common issue so I checked a few SO posts but none of them solved my problem.
Here is my Dockerfile:
# MkDocs container
FROM python:3-alpine AS build-env
RUN apk add bash
RUN pip install --upgrade pip
RUN pip install pymdown-extensions \
&& pip install mkdocs \
&& pip install mkdocs-material \
&& pip install mkdocs-rtd-dropdown \
&& pip install mkdocs-git-revision-date-plugin \
&& pip install mkdocs-git-revision-date-localized-plugin \
&& pip install mkdocs-redirects
# executed at ~/Developer/MkDocs
RUN mkdir -p /home/mkdocs/
WORKDIR /home/mkdocs/
COPY . .
RUN mkdocs build -s
WORKDIR /
# Nginx container
FROM nginx:1.21.6-alpine
RUN apk add bash
EXPOSE 80
RUN cat /etc/nginx/nginx.conf
COPY nginx.conf /etc/nginx/nginx.conf
WORKDIR /
RUN mkdir -p /home/mkdocs
COPY --from=build-env /home/mkdocs/site/ /home/mkdocs/
RUN mv /home/mkdocs/* /usr/share/nginx/html/
RUN chown nginx:nginx /usr/share/nginx/html/*
USER nginx:nginx
And here is the command to run docker:
docker run -it --name mkdocs -p 8789:80 nginx
Running localhost:8789 only shows the default nginx homepage, not the built one of MkDocs. I also run docker exec -it --user root <PID> bash to check the directory /usr/share/nginx/html/ but the copied files are not there.
My other checks:
First, I'm 100% sure that the files built in the first stage works and exists
Second, this is what completely frustrated me out. If I run the docker using docker run -it --entrypoint=/bin/bash mkdocs:v1, I can actually see the built MkDocs files:
bash-5.1$ ls /usr/share/nginx/html
404.html 50x.html assets index.html search sitemap.xml sitemap.xml.gz
bash-5.1$