Improve build times by sharing cache and allowing platform selection

Dockerfile

@@ -1,16 +1,24 @@
-FROM golang:1.16-buster AS builder
+# All builds should be done using the platform native to the build node to allow
+#  cache sharing of the go mod download step.
+# Go cross compilation is also faster than emulation the go compilation across
+#  multiple platforms.
+FROM --platform=${BUILDPLATFORM} golang:1.16-buster AS builder
 
 # Copy sources
 WORKDIR $GOPATH/src/github.com/oauth2-proxy/oauth2-proxy
 
 # Fetch dependencies
 COPY go.mod go.sum ./
-RUN GO111MODULE=on go mod download
+RUN go mod download
 
 # Now pull in our code
 COPY . .
 
+# Arguments go here so that the previous steps can be cached if no external
+#  sources have changed.
 ARG VERSION
+ARG TARGETPLATFORM
+ARG BUILDPLATFORM
 
 # Build binary and make sure there is at least an empty key file.
 #  This is useful for GCP App Engine custom runtime builds, because
@@ -18,7 +26,15 @@ ARG VERSION
 #  build the key into the container and then tell it where it is
 #  by setting OAUTH2_PROXY_JWT_KEY_FILE=/etc/ssl/private/jwt_signing_key.pem
 #  in app.yaml instead.
-RUN VERSION=${VERSION} make build && touch jwt_signing_key.pem
+# Set the cross compilation arguments based on the TARGETPLATFORM which is
+#  automatically set by the docker engine.
+RUN case ${TARGETPLATFORM} in \
+         "linux/amd64")  GOARCH=amd64  ;; \
+         "linux/arm64")  GOARCH=arm64  ;; \
+         "linux/arm/v6") GOARCH=arm GOARM=6  ;; \
+    esac && \
+    printf "Building OAuth2 Proxy for arch ${GOARCH}\n" && \
+    VERSION=${VERSION} make build && touch jwt_signing_key.pem
 
 # Copy binary to alpine
 FROM alpine:3.14