This patch implements `docker build --secret id=mysecret,src=/secret/file`
for buildkit frontends that request the mysecret secret.
It is currently implemented in the tonistiigi/dockerfile:secrets20180808
frontend via RUN --mount=type=secret,id=mysecret
Signed-off-by: Tibor Vass <tibor@docker.com>
Unfortunately, this is for now the only way to see the output of RUN commands when using buildkit.
It is equivalent to `DOCKER_BUILDKIT=1 docker build . 2>&1 | cat`
Signed-off-by: Tibor Vass <tibor@docker.com>