Build via Dockerfile fails on ppc64le

### Description

The manual build via Dockerfile fails on ppc64le for v1.126.0 with the following error:

_[1/4] STEP 1/8: FROM ghcr.io/astral-sh/uv:python3.12-bookworm AS requirements
Trying to pull ghcr.io/astral-sh/uv:python3.12-bookworm...
Error: creating build container: choosing an image from manifest list docker://ghcr.io/astral-sh/uv:python3.12-bookworm: no image found in image index for architecture ppc64le, variant "", OS linux_

The following pull request seems to be the reason since the last version v1.125.0 still worked out: https://github.com/element-hq/synapse/pull/18038

The new Dockerfile seems to indicate it was written for "both" architectures amd64 and arm64, but does not seem to be "platform-agnostic" anymore.

Are there any changes or build arguments I can try to build successfully again?

### Steps to reproduce

- podman build -t debian/synapse-v1.126.0 -f docker/Dockerfile .

### Homeserver

another homeserver

### Synapse Version

v1.126.0

### Installation Method

Other (please mention below)

### Database

PostgreSQL

### Workers

Single process

### Platform

Debian Bookworm (ppc64le)

### Configuration

_No response_

### Relevant log output

```shell
[1/4] STEP 1/8: FROM ghcr.io/astral-sh/uv:python3.12-bookworm AS requirements
Trying to pull ghcr.io/astral-sh/uv:python3.12-bookworm...
Error: creating build container: choosing an image from manifest list docker://ghcr.io/astral-sh/uv:python3.12-bookworm: no image found in image index for architecture ppc64le, variant "", OS linux
```

### Anything else that would be useful to know?

_No response_