build: add release-install goal, hash individual binaries

We add a new make goal called release-install that creates the same
reproducible binaries as the release script would create, but only for
the current OS/architecture. It then installs those binaries to the
system's GOBIN directory.
To allow easy verification of individual binaries (instead of just the
packag tarballs/zips), we also add the hashes of lnd and lncli binaries
to the manifest. We do the same in the docker build.

Dockerfile

@@ -23,8 +23,7 @@ RUN apk add --no-cache --update alpine-sdk \
 &&  git clone https://github.com/lightningnetwork/lnd /go/src/github.com/lightningnetwork/lnd \
 &&  cd /go/src/github.com/lightningnetwork/lnd \
 &&  git checkout $checkout \
-&&  make \
-&&  make install tags="signrpc walletrpc chainrpc invoicesrpc"
+&&  make release-install
 
 # Start a new, final image.
 FROM alpine as final
@@ -42,6 +41,11 @@ RUN apk --no-cache add \
 COPY --from=builder /go/bin/lncli /bin/
 COPY --from=builder /go/bin/lnd /bin/
 
+# Store the SHA256 hash of the binaries that were just produced for later
+# verification.
+RUN sha256sum /bin/lnd /bin/lncli > /shasums.txt \
+  && cat /shasums.txt
+
 # Expose lnd ports (p2p, rpc).
 EXPOSE 9735 10009