build: bot パッケージのディレクトリを WORKDIR に指定

[MikuroXina]

close #1229.

Type of Change:

Dockerfile の修正

Details of implementation (実施内容)

ワーキングディレクトリが変化したために動作しなくなっていたので, WORKDIR で改めて指定するようにしました. もっといいやり方があるかもしれません.

[github-actions[bot]]

Hadolint: success

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (c2485f2) 56.66% compared to head (f751210) 56.66%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1230 +/- ## ======================================= Coverage 56.66% 56.66% ======================================= Files 103 103 Lines 7015 7015 Branches 515 515 ======================================= Hits 3975 3975 Misses 2526 2526 Partials 514 514 ``` | [Flag](https://app.codecov.io/gh/approvers/OreOreBot2/pull/1230/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=approvers) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/approvers/OreOreBot2/pull/1230/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=approvers) | `56.66% <ø> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=approvers#carryforward-flags-in-the-pull-request-comment) to find out more.

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