Add note for Debian family(such as Ubuntu)

[eromoe]

There are many requests that demand using /bin/bash instead of /bin/sh

#8100
#7489
#7281

It because they are using the image of Debian family. It is inconvenient to write all command like RUN ["/bin/bash", "-c", "python test.py"].
And here is a much more simple way:

FROM some-image
RUN rm /bin/sh && ln -s /bin/bash /bin/sh

I think this is a very useful note should be include in docs.

[GordonTheTurtle]

Please sign your commits following these rules:
https://github.com/docker/docker/blob/master/CONTRIBUTING.md#sign-your-work
The easiest way to do this is to amend the last commit:

$ git clone -b "patch-1" git@github.com:eromoe/docker.git somewhere
$ cd somewhere
$ git commit --amend -s --no-edit
$ git push -f

Amending updates the existing PR. You DO NOT need to open a new one.

[AkihiroSuda]

I'm -1 on this change.
Users should be able to use SHELL instead.
https://docs.docker.com/engine/reference/builder/#/shell

[ijc]

Apart from the suggestion to use SHELL (which I agree with) the better/supported way to change the default /bin/sh link in Debian from dash to bash would be something like:

RUN echo "dash dash/sh boolean false" | debconf-set-selections - && dpkg-reconfigure dash

and to switch back:

RUN echo "dash dash/sh boolean true" | debconf-set-selections - && dpkg-reconfigure dash

(only tried this from the shell prompt not actually in a Dockerfile so caveat emptor)