distribution: create sha256sum.txt file when creating binaries to allow validation of checksums.

* update README.md to include instructions on how to verify prebuilt binaries for new releases.

Author: talam

README.md

@@ -16,6 +16,11 @@ to validate accounts by email, domain or group.
 ## Installation
 
 1. Download [Prebuilt Binary](https://github.com/bitly/oauth2_proxy/releases) (current release is `v2.2`) or build with `$ go get github.com/bitly/oauth2_proxy` which will put the binary in `$GOROOT/bin`
+Prebuilt binaries can be validated by extracting the file and verifying it against the `sha256sum.txt` checksum file provided for each release starting with version `v2.3`.
+```
+sha256sum -c sha256sum.txt 2>&1 | grep OK
+oauth2_proxy-2.3.linux-amd64: OK
+```
 2. Select a Provider and Register an OAuth Application with a Provider
 3. Configure OAuth2 Proxy using config file, command line options, or environment variables
 4. Configure SSL or Deploy behind a SSL endpoint (example provided for Nginx)

dist.sh

@@ -13,6 +13,7 @@ os=$(go env GOOS)
 arch=$(go env GOARCH)
 version=$(cat $DIR/version.go | grep "const VERSION" | awk '{print $NF}' | sed 's/"//g')
 goversion=$(go version | awk '{print $3}')
+sha256sum=()
 
 echo "... running tests"
 ./test.sh
@@ -25,10 +26,22 @@ for os in windows linux darwin; do
     fi
     BUILD=$(mktemp -d ${TMPDIR:-/tmp}/oauth2_proxy.XXXXXX)
     TARGET="oauth2_proxy-$version.$os-$arch.$goversion"
+    FILENAME="oauth2_proxy-$version.$os-$arch$EXT"
     GOOS=$os GOARCH=$arch CGO_ENABLED=0 \
-        go build -ldflags="-s -w" -o $BUILD/$TARGET/oauth2_proxy$EXT || exit 1
-    pushd $BUILD
-    tar czvf $TARGET.tar.gz $TARGET
+        go build -ldflags="-s -w" -o $BUILD/$TARGET/$FILENAME || exit 1
+    pushd $BUILD/$TARGET
+    sha256sum+=("$(shasum -a 256 $FILENAME || exit 1)")
+    cd .. && tar czvf $TARGET.tar.gz $TARGET
     mv $TARGET.tar.gz $DIR/dist
     popd
 done
+
+checksum_file="sha256sum.txt"
+cd $DIR/dist
+if [ -f $checksum_file ]; then
+    rm $checksum_file
+fi
+touch $checksum_file
+for checksum in "${sha256sum[@]}"; do
+    echo "$checksum" >> $checksum_file
+done