Simplify Dockerfile to use pre-built binary from GoReleaser

GoReleaser builds the binary and copies it into the Docker build context,
so the multi-stage Go compilation is unnecessary and breaks the build
(go.sum not present in the context).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: nficano

Dockerfile

@@ -1,25 +1,3 @@
-FROM golang:1.22-alpine AS builder
-
-RUN apk add --no-cache git ca-certificates
-
-WORKDIR /src
-
-COPY go.mod go.sum ./
-RUN go mod download
-
-COPY . .
-
-ARG VERSION=dev
-ARG COMMIT=unknown
-ARG DATE=unknown
-
-RUN CGO_ENABLED=0 go build \
-    -ldflags "-s -w \
-      -X github.com/nficano/github-runner/internal/version.Version=${VERSION} \
-      -X github.com/nficano/github-runner/internal/version.Commit=${COMMIT} \
-      -X github.com/nficano/github-runner/internal/version.Date=${DATE}" \
-    -o /github-runner ./cmd/github-runner
-
 FROM alpine:3.19
 
 RUN apk add --no-cache \
@@ -31,7 +9,7 @@ RUN apk add --no-cache \
     && addgroup -S runner \
     && adduser -S runner -G runner
 
-COPY --from=builder /github-runner /usr/local/bin/github-runner
+COPY github-runner /usr/local/bin/github-runner
 
 RUN mkdir -p /etc/github-runner /var/lib/github-runner /var/log/github-runner \
     && chown -R runner:runner /var/lib/github-runner /var/log/github-runner