ci: java container build to use go.mod at the root (#2679)

After the migration from sdk-platform-java to librarian, we start
to use the go.mod at the root of the librarian repository.
This is the same setup as the automation's dispatcher (
infra/dispatcher/Dockerfile).

I confirmed that the Dockerfile works:

```
suztomo@suztomo:~/librarian$ docker build -f internal/container/java/Dockerfile .
[+] Building 21.1s (14/14) FINISHED                                                                                                        docker:default

...(omit)...

 => [builder 6/7] COPY . .                                                                                                                           0.7s
 => [builder 7/7] RUN CGO_ENABLED=0 GOOS=linux go build -v -o /librariangen ./internal/container/java                                               14.0s
 => [stage-1 2/2] COPY --from=builder /librariangen /usr/local/bin/librariangen                                                                      0.1s
 => exporting to image                                                                                                                               0.1s
 => => exporting layers                                                                                                                              0.0s
 => => writing image sha256:4dd4813eeea24c268d638011d961641a11d31014b5abbc399b20346602424fae
```

Author: suztomo

internal/container/java/Dockerfile

@@ -50,7 +50,7 @@ WORKDIR /src
 COPY . .
 
 # Build the librariangen binary.
-RUN CGO_ENABLED=0 GOOS=linux go build -v -o /librariangen .
+RUN CGO_ENABLED=0 GOOS=linux go build -v -o /librariangen ./internal/container/java
 
 # --- Final Stage ---
 # This stage creates the final, minimal image with the compiled binary and
@@ -62,4 +62,4 @@ COPY --from=builder /librariangen /usr/local/bin/librariangen
 
 # Set the entrypoint for the container to run the compiled librariangen.
 # The Librarian will provide commands like 'generate' as arguments.
-ENTRYPOINT [ "/usr/local/bin/librariangen" ]
+ENTRYPOINT [ "/usr/local/bin/librariangen" ]

internal/container/java/cloudbuild-exitgate.yaml

@@ -23,8 +23,15 @@ steps:
       - |
         docker pull us-central1-docker.pkg.dev/cloud-sdk-librarian-prod/images-dev/librarian-java:latest || exit 0
   - name: gcr.io/cloud-builders/docker
-    args: ["build", "--cache-from", "us-central1-docker.pkg.dev/cloud-sdk-librarian-prod/images-dev/librarian-java:latest", "-t", "us-central1-docker.pkg.dev/cloud-sdk-librarian-prod/images-dev/librarian-java:latest", "-f", "Dockerfile", "."]
-    dir: internal/container/java
+    args:
+      - "build"
+      - "--cache-from"
+      - "us-central1-docker.pkg.dev/cloud-sdk-librarian-prod/images-dev/librarian-java:latest"
+      - "-t"
+      - "us-central1-docker.pkg.dev/cloud-sdk-librarian-prod/images-dev/librarian-java:latest"
+      - "-f"
+      - "internal/container/java/Dockerfile"
+      - "."
 options:
   machineType: 'E2_HIGHCPU_8'
   requestedVerifyOption: VERIFIED # For provenance attestation generation