Update to latest alpine base for go-compile

I noticed that running the runes in docs/vendoring.md made a bunch of files
which are omitted by modern vndr come back, presumably due to the old version
of vndr referenced there. So update the go-compile package to the current
version everywhere.

Also correct the use of `make` variable syntax (`$(PWD)`) where shell syntax
command substitution for `pwd` was desired.

Signed-off-by: Ian Campbell <ijc@docker.com>

tools/go-compile/Dockerfile

@@ -1,4 +1,5 @@
-FROM linuxkit/alpine:9bcf61f605ef0ce36cc94d59b8eac307862de6e1 AS mirror
+FROM linuxkit/alpine:3744607156e6b67e3e7d083b15be9e7722215e73 AS mirror
+
 RUN mkdir -p /out/etc/apk && cp -r /etc/apk/* /out/etc/apk/
 RUN apk add --no-cache --initdb -p /out \
     alpine-baselayout \