Add Dockerfile and build with GitHub Actions

Dockerfile

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+FROM golang:1.21-alpine
+
+WORKDIR /usr/src/app
+
+# pre-copy/cache go.mod for pre-downloading dependencies and only redownloading them in subsequent builds if they change
+COPY go.mod go.sum ./
+RUN go mod download && go mod verify
+
+COPY . .
+RUN go build -v -o /usr/local/bin/app ./...
+
+CMD ["app"]