Support SIYUAN_WORKSPACE_PATH in Docker

[luke-zhu]

Feature or bug? 特性或者缺陷？

This change makes SIYUAN_WORKSPACE_PATH env variable
update the WORKSPACE_DIR variable in the Docker entrypoint
shell script.

Multilingual or copywriting? 多语言或者文案？

N/A

Dev branch!

Any changes should be submitted to the dev branch.
任何改动，请提交到 dev 分支。

[github-actions]

Your PR was set to target master, PRs should be target dev
The base branch of this PR has been automatically changed to dev, please check that there are no merge conflicts

[luke-zhu]

I have tested using the following config:

In the old PR the env variable handling was bugged. And it seems like -v is not supported so I use -n.

WORKSPACE_DIR can be set at the top of the file instead too:

WORKSPACE_DIR=${SIYUAN_WORKSPACE_PATH:-"/siyuan/workspace"}

This saves 3 lines of code but affects code distance between --workspace and SIYUAN_WORKSPACE_PATH.

[luke-zhu]

And reverting back still works:

[88250]

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Thank you for your contribution. SiYuan will be more wonderful with you!