Copy root .go files to the builder image in the Dockerfile.

This fixes the image build error where, after having changed the build
command to use the root of the repo instead of ./src, the Go toolchain
is unable to find any .go files to build.

Dockerfile

@@ -21,6 +21,7 @@ RUN apk --no-cache add git
 WORKDIR /build
 COPY go.mod go.sum ./
 RUN go mod download
+COPY *.go ./
 COPY src/ ./src/
 RUN go build -ldflags "-s -w" -o git-pages .