Docker ADD doesn't extract file

Question

I'm trying to download a specific Node version and copy the binary from the extracted tar.xz, but it doesn't look like ADD actually extracts the file.

This is my Dockerfile:

FROM arm32v7/debian:stretch-slim

ENV nodever="v6.10.0"
ADD https://nodejs.org/dist/${nodever}/node-${nodever}-linux-armv7l.tar.xz /
COPY /node-${nodever}-linux-armv7l/bin/node /

RUN cat /etc/issue && \
\
apk update &&  \
apk add \
    dnsmasq \
    hostapd \
    iptables

RUN mkdir app
ADD start.sh /app/start.sh
ADD server /app/server

ADD configs/hostapd.conf /etc/hostapd.conf
ADD configs/dnsmasq.conf /etc/dnsmasq.conf

WORKDIR /app
CMD ["/app/start.sh"]

but running docker -v build yields this error:

Sending build context to Docker daemon  9.728kB
Step 1/12 : FROM arm32v7/debian:stretch-slim

---> 1510cdc40eca
Step 2/12 : ENV nodever "v6.10.0"

---> Using cache
---> 1ca3d2f72a3c
Step 3/12 : ADD https://nodejs.org/dist/${nodever}/node-${nodever}-linux-armv7l.tar.xz /
Downloading   8.34MB/8.34MB

---> a1fefb9bda66
Step 4/12 : COPY /node-${nodever}-linux-armv7l/bin/node /
COPY failed: stat /var/lib/docker/tmp/docker-builder233292954/node-v6.10.0-linux-armv7l/bin/node: no such file or directory

I've tried running RUN ls / which shows that it has downloaded the archive but it isn't extracted.

Answer 1

This is documented:

If is a local tar archive in a recognized compression format (identity, gzip, bzip2 or xz) then it is unpacked as a directory.

Notice the emphasis on "local".

This was actually changed and reverted at some point, so it's unlikely this will ever change again.