Fix scope of user srv build in rolldice example

[MrAlias]

Fix #2889

> docker-compose build --no-cache user
[+] Building 65.8s (14/14) FINISHED
 => [internal] load local bake definitions                                                                                                                               0.0s
 => => reading from stdin 580B                                                                                                                                           0.0s
 => [internal] load build definition from Dockerfile                                                                                                                     0.0s
 => => transferring dockerfile: 645B                                                                                                                                     0.0s
 => [internal] load metadata for docker.io/library/golang:1.25.1@sha256:d7098379b7da665ab25b99795465ec320b1ca9d4addb9f77409c4827dc904211                                 0.0s
 => [internal] load .dockerignore                                                                                                                                        0.0s
 => => transferring context: 2B                                                                                                                                          0.0s
 => [stage-0 1/7] FROM docker.io/library/golang:1.25.1@sha256:d7098379b7da665ab25b99795465ec320b1ca9d4addb9f77409c4827dc904211                                           0.0s
 => [internal] load build context                                                                                                                                        0.0s
 => => transferring context: 2.56kB                                                                                                                                      0.0s
 => CACHED [stage-0 2/7] WORKDIR /usr/src/go.opentelemetry.io/auto/examples/rolldice/user                                                                                0.0s
 => [stage-0 3/7] COPY ./sdk /usr/src/go.opentelemetry.io/auto/sdk                                                                                                       0.1s
 => [stage-0 4/7] COPY ./examples/rolldice/user/go.mod ./examples/rolldice/user/go.sum ./                                                                                0.1s
 => [stage-0 5/7] RUN --mount=type=cache,target=/go/pkg go mod download && go mod verify                                                                                 1.7s
 => [stage-0 6/7] COPY ./examples/rolldice/user/ ./                                                                                                                      0.1s
 => [stage-0 7/7] RUN --mount=type=cache,target=/root/.cache/go-build     --mount=type=cache,target=/go/pkg     go build -v -o /usr/local/bin/user ./server/...         59.9s
 => exporting to image                                                                                                                                                   1.9s
 => => exporting layers                                                                                                                                                  1.9s
 => => writing image sha256:aa446b86902b8b29f0c37083d00a22643fc36cd01567c1afc037bf3b029105dd                                                                             0.0s
 => => naming to docker.io/library/rolldice-user                                                                                                                         0.0s
 => resolving provenance for metadata file                                                                                                                               0.0s
[+] Building 1/1
 ✔ rolldice-user  Built