docker build read from stdin needs documentation

[thaJeztah]

Problem description

Docker 17.05 added an option to perform docker build with the Dockerfile piped through stdin, but being able to use local (or remote) build-context. Prior to docker 17.05, building an image with a Dockerfile from stdin did not send the build-context.

Docker < 17.05;

docker build -t foo -<<EOF
FROM busybox
RUN echo "hello world"
EOF

Docker 17.05 and up;

docker build -t . -f-<<EOF
FROM busybox
RUN echo "hello world"
COPY . /my-copied-files
EOF

or (remote build-context);

docker build -t foo https://github.com/thajeztah/pgadmin4-docker.git -f-<<EOF
FROM busybox
COPY LICENSE config_local.py /usr/local/lib/python2.7/site-packages/pgadmin4/
EOF

(or other variations to get the Dockerfile from stdin)

The old syntax is still supported; the new one is just added

This functionality wasn't documented when the pull request was merged, so needs an update (probably both in CLI reference and other pages)

/cc @dsheets @tonistiigi @mstanleyjones

Related pull request: moby/moby#31236
And proposal: moby/moby#27393

[amontalban]

Would love to have this documented, found this googling around and it's a cool feature that should be known. Thanks for being awesome guys!

[mdlinville]

@thaJeztah Sorry I didn't get to this. You might want to tackle this in docker/cli instead of here.

[docker-robott]

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