Add some comments in Dockerfile

Dockerfile

+# Dockerfile for Hugo (HUGO=hugo) / Hugo Extended (HUGO=hugo_extended)
+# HUGO_VERSION / HUGO_SHA / HUGO_EXTENDED_SHA is automatically updated
+# by update.py when new release is available on the upstream.
+# Utilize multi-stage builds to make images optimized in size.
+
+# First stage - download prebuilt hugo binary from the GitHub release.
+# Use golang image to run https://github.com/yaegashi/muslstack
+# on hugo executable to extend its default thread stack size to 8MB
+# to work around segmentation fault issues.
 FROM golang:1.12-alpine
 ARG HUGO=hugo
 ARG HUGO_VERSION=0.55.6
@@ -15,6 +24,8 @@ RUN set -eux && \
 RUN go get github.com/yaegashi/muslstack
 RUN muslstack -s 0x800000 /usr/bin/hugo
 
+# Second stage - build the final image with minimal apk dependencies.
+# alpine:edge is required for muslstack to work as of June 2019.
 FROM alpine:edge
 ARG HUGO=hugo
 COPY --from=0 /usr/bin/hugo /usr/bin