fix: remove duplicated build type in Dockerfile (#1598)

## Description

---
Removing duplication of `container.`.

## PR Checklist

---
> Thank you for opening a Pull Request! Before submitting your PR, there
are a
> few things you can do to make sure it goes smoothly:

- [x] Make sure you reviewed

[CONTRIBUTING.md](https://github.com/googleapis/genai-toolbox/blob/main/CONTRIBUTING.md)
- [x] Make sure to open an issue as a

[bug/issue](https://github.com/googleapis/genai-toolbox/issues/new/choose)
before writing your code! That way we can discuss the change, evaluate
  designs, and agree on the general idea
- [x] Ensure the tests and linter pass
- [x] Code coverage does not decrease (if any source code was changed)
- [x] Appropriate docs were updated (if necessary)
- [x] Make sure to add `!` if this involve a breaking change

🛠️ Fixes #1550

Dockerfile

@@ -25,7 +25,7 @@ ARG COMMIT_SHA=""
 
 RUN go get ./...
 RUN CGO_ENABLED=0 GOOS=${TARGETOS} GOARCH=${TARGETARCH} \
-    go build -ldflags "-X github.com/googleapis/genai-toolbox/cmd.buildType=container.${BUILD_TYPE} -X github.com/googleapis/genai-toolbox/cmd.commitSha=${COMMIT_SHA}"
+    go build -ldflags "-X github.com/googleapis/genai-toolbox/cmd.buildType=${BUILD_TYPE} -X github.com/googleapis/genai-toolbox/cmd.commitSha=${COMMIT_SHA}"
 
 # Final Stage
 FROM gcr.io/distroless/static:nonroot