Fix build breaking due to unpinned poetry

[loneil]

Poetry released a major version 2.0. We weren't pinning Poetry in the dockerfile so it just gets the latest one and that now errors with:

4.878 Installing the current project: acapy-vc-authn-oidc (0.2.2)
4.880
4.880 Warning: The current project could not be installed: No file/folder found for package acapy-vc-authn-oidc
4.880 If you do not want to install the current project use --no-root.
4.880 If you want to use Poetry only for dependency management but not for packaging, you can disable package mode by setting package-mode = false in your pyproject.toml file.
4.880 If you did intend to install the current project, you may need to set `packages` in your pyproject.toml file.
4.880
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Dockerfile:9
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   7 |
   8 |     COPY pyproject.toml poetry.lock README.md ./
   9 | >>> RUN poetry install --only main
  10 |
  11 |     COPY ./oidc-controller .
--------------------
ERROR: failed to solve: process "/bin/sh -c poetry install --only main" did not complete successfully: exit code: 1

Specify Poetry version to fix this.

Will look at fix and any other enhancements for new Poetry in
#701

[coveralls]

Pull Request Test Coverage Report for Build 12641055638

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 85.254%

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Totals
Change from base Build 12416479364:	0.0%
Covered Lines:	688
Relevant Lines:	807

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💛 - Coveralls