Add a Dockerfile

This way I can use it a Kubernetes cronjob that needs to POST to an
IAP enabled endpoint :)

example usage:

```
$ docker build . -t iap_curl
$ docker run -v ~/Downloads:/data -e
GOOGLE_APPLICATION_CREDENTIALS=/data/cool-proj-b96f2ba9d683.json -e
IAP_CLIENT_ID=999991111111-asdfdeabdefedeadbeef123456789nkl.apps.googleusercontent.com
--rm -it iap_curl -X POST https://mycoolapp.com/v1/rentals/promote
```

Author: joshk0

Dockerfile

@@ -0,0 +1,48 @@
+FROM golang:alpine as builder
+
+# Install git + SSL ca certificates.
+# Git is required for fetching the dependencies.
+# Ca-certificates is required to call HTTPS endpoints.
+# gcc and musl-dev required by go-sqlite3
+RUN apk update && apk add --no-cache git ca-certificates tzdata gcc musl-dev && update-ca-certificates
+
+WORKDIR /app
+
+# Fetch dependencies:
+# https://medium.com/@petomalina/using-go-mod-download-to-speed-up-golang-docker-builds-707591336888
+COPY go.mod .
+COPY go.sum .
+RUN go mod download
+
+COPY . .
+
+ENV GOOS=linux
+
+# Multi-stage build based off:
+# https://github.com/chemidy/smallest-secured-golang-docker-image
+
+# Run tests
+RUN go test -cover ./...
+
+# Build the binary
+RUN go build -ldflags="-w -s" -a -installsuffix cgo -o /go/bin/app .
+
+# Create app user
+RUN adduser -D -g '' app
+
+FROM alpine
+
+# iap_curl calls out to curl after establishing the token
+RUN apk update && apk add --no-cache curl
+
+# Import from builder
+COPY --from=builder /usr/share/zoneinfo /usr/share/zoneinfo
+COPY --from=builder /etc/ssl/certs/ca-certificates.crt /etc/ssl/certs/
+COPY --from=builder /etc/passwd /etc/passwd
+
+COPY --from=builder /go/bin/app /go/bin/app
+
+# Use an unprivileged user.
+USER app
+
+ENTRYPOINT ["/go/bin/app"]