Pin docker buildx to fix OCI issue on CF

[ryantk]

Changes in this PR

Having problems deploying due to the following errors:

202***-0***-2***T***0:06:08.***8+0000 [STG/0] ERR Failed getting docker image manifest by tag: Error reading manifest 805a248 in ghcr.io/dfe-digital/buy-for-your-school: manifest unknown: OCI index found, but Accept header does not support OCI indexes  Going to retry attempt: ***
	202***-0***-2***T***0:06:08.77+0000 [STG/0] ERR Failed getting docker image manifest by tag: Error reading manifest 805a248 in ghcr.io/dfe-digital/buy-for-your-school: manifest unknown: OCI index found, but Accept header does not support OCI indexes  Going to retry attempt: 2
	202***-0***-2***T***0:06:09.***9+0000 [STG/0] ERR Failed getting docker image manifest by tag: Error reading manifest 805a248 in ghcr.io/dfe-digital/buy-for-your-school: manifest unknown: OCI index found, but Accept header does not support OCI indexes  Going to retry attempt: ***
	202***-0***-2***T***0:06:09.99+0000 [STG/0] ERR Failed getting docker image manifest by tag: Error reading manifest 805a248 in ghcr.io/dfe-digital/buy-for-your-school: manifest unknown: OCI index found, but Accept header does not support OCI indexes

Fix suggested here: DFE-Digital/get-into-teaching-app#3089

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QA application accessible at: https://ghbs-qa-3.london.cloudapps.digital