build: use pre-built binary in Dockerfile

GoReleaser copies the compiled binary into the Docker context.
Dockerfile should COPY the binary, not rebuild from source.

Dockerfile

@@ -1,14 +1,5 @@
-FROM golang:1.26-alpine AS builder
-ARG TARGETARCH
-RUN apk add --no-cache git make
-WORKDIR /src
-COPY go.mod go.sum ./
-COPY vendor/ vendor/
-COPY . .
-RUN CGO_ENABLED=0 GOOS=linux GOARCH=${TARGETARCH:-amd64} go build -mod=vendor -o /mpcd ./cmd/mpcd/
-
 FROM alpine:3.20
 RUN apk add --no-cache ca-certificates
-COPY --from=builder /mpcd /usr/local/bin/mpcd
+COPY mpcd /usr/local/bin/mpcd
 EXPOSE 8081
 ENTRYPOINT ["mpcd"]