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

## Description

---
Removing duplication of `container.`.

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🛠️ Fixes googleapis#1550

Author: Yuan325

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