1.12.12 regression: Dockerfile doesn't build – "/apps/web/src/customisations/README.md": not found

### Steps to reproduce

```
$ git status
HEAD detached at v1.12.12
nothing to commit, working tree clean

$ docker build --no-cache --pull -f apps/web/Dockerfile .
```

### Outcome

#### What did you expect?

A successful Docker build, as in previous versions.

#### What happened instead?

```
=> ERROR [builder 3/8] COPY --exclude=docker . /src 0.0s
------
> [builder 3/8] COPY --exclude=docker . /src:
------
Dockerfile:13
--------------------
11 | WORKDIR /src
12 |
13 | >>> COPY --exclude=docker . /src
14 | RUN corepack enable
15 | RUN /src/scripts/docker-link-repos.sh
--------------------
ERROR: failed to solve: failed to compute cache key: failed to calculate checksum of ref af93a94e-b8ad-4377-bb2a-ee82460f7175::r150tzhzedxh7revzar5uafg4: "/apps/web/src/customisations/README.md": not found
```

### Operating system

Ubuntu 24.04.4, Docker version 28.2.2, build 28.2.2-0ubuntu1~24.04.1

### Browser information

n/a

### URL for webapp

n/a

### Application version

1.12.12

### Homeserver

n/a

### Will you send logs?

see above