Update solidnerd/bookstack Docker tag to v21.12.1

This MR contains the following updates:

Package	Type	Update	Change
solidnerd/bookstack	Kustomization	minor	21.11.3 -> 21.12.1

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Release Notes

solidnerd/docker-bookstack

v21.12.1

Compare Source

• 52dc6b2 - chore: update to 21.12.1 (#​293)
• 4611ab7 - Add APP_URL to dockercompose and README.md (#​292)

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