Add build hint to Dockerfile

Dockerfile

@@ -1,4 +1,10 @@
-# This assumes binaries are present, see COPY directive.
+# This Dockerfile is infused with magic to speedup the build.
+# In particular, it requires built binaries to be present (see COPY directive).
+#
+# tl;dr: To make this build work, run
+#   ./dev/focusdev build
+# and find your freshly built images tagged with the `localbuild` tag.
+
 FROM alpine AS chmodder
 ARG TARGETARCH
 COPY /artifacts/binaries-$TARGETARCH/focus /app/