Doc: Add the document of backend E2E environment

[Jaycean]

Feature request

Please describe your feature

• In the process of submitting my contribution, I found that there was no local documentation for setting up E2E test environment, and I didn't know how to conduct E2E test locally. I could only view the results of GitHub Ci by submitting code, which was very unfriendly. So I checked the command of GitHub Ci and found that I needed to run docker upstream locally

Describe the solution you'd like

• I want to submit a PR update doc, so that more developers can quickly develop for the first time and use E2E test locally.

docker run -d --name upstream -v /home/apisix-dashboard/api/test/docker/upstream.conf:/etc/nginx/conf.d/default.conf:ro -p 80:80 -p 1980:1980 -p 1981:1981 -p 1982:1982 -p 1983:1983 -p 1984:1984 johz/upstream:v2.0

[imjoey]

@Jaycean good catch. I believe that would be very helpful for every contributor. Your PR is greatly welcome. Thanks.

[juzhiyuan]

cc @starsz @LiteSun to know this tip.

[starsz]

Can't agree anymore. 😄