github: fix dockerfile (#2486)

Fix `Dockerfile` by removing `\` symbols.

This fixes the `build-push-deploy` github workflow which is failing: https://github.com/ObolNetwork/charon/actions/runs/5689854878/job/15422120453

category: fixbuild 
ticket: none

Dockerfile

@@ -13,8 +13,8 @@ RUN echo "Building with GO_BUILD_FLAG='${GO_BUILD_FLAG}'"
 RUN \
    --mount=type=cache,target=/go/pkg \
    --mount=type=cache,target=/root/.cache/go-build \
-   go build -o charon "${GO_BUILD_FLAG}" . \
-RUN echo "Built charon version=$(./charon version)" \
+   go build -o charon "${GO_BUILD_FLAG}" .
+RUN echo "Built charon version=$(./charon version)"
 
 # Copy final binary into light stage.
 FROM debian:bullseye-slim