Add "CMD"

[tianon]

This was rejected previously (#60) on the basis of keeping the layer count to an absolute minimum. Since Docker 1.10+, instructions like "CMD" simply modify metadata and no longer create an additional layer that needs to be transported at pull time. I'm hoping that means we can reconsider it? 😇

Closes #193

[bboreham]

What's the maintainers' view on this? It's my last hope for simpler demos now my 'shell' PR is closed.

(tests appear to have failed on a version check, @tianon)

[andyshinn]

I'm not opposed to it. Though, I'd be curious if there are implications where people use bare ENTRYPOINTs and then suddenly get a CMD added to that.

If users have an ENTRYPOINT and this gets added and they don't explicitly override the CMD, will it be an issue?

[tianon]

We actually ran into that enough early on that Docker resets any inherited CMD when it encounters ENTRYPOINT 👍

To illustrate:

$ docker build -

FROM alpine:3.4
CMD ["/bin/sh"]

<Ctrl+D>

Sending build context to Docker daemon 2.048 kB
Step 1 : FROM alpine:3.4
 ---> ee4603260daa
Step 2 : CMD /bin/sh
 ---> Running in c90072616302
 ---> 3da1cf08278a
Removing intermediate container c90072616302
Successfully built 3da1cf08278a
$ docker build -

FROM 3da1cf08278a
ENTRYPOINT ["echo", "hi"]

<Ctrl+D>

Sending build context to Docker daemon 2.048 kB
Step 1 : FROM 3da1cf08278a
 ---> 3da1cf08278a
Step 2 : ENTRYPOINT echo hi
 ---> Running in 59902a5804e8
 ---> f4fd19af8fff
Removing intermediate container 59902a5804e8
Successfully built f4fd19af8fff
$ docker run --rm f4fd19af8fff
hi

(simply hi as opposed to hi /bin/sh)

[andyshinn]

I'm not opposed seeing as how Docker has evolved in the way it handles layers. It also seems like something people want. Ping @gliderlabs/maintainers for someone else to also approve.

[ncopa]

I am all for this 👍

[andyshinn]

Let's get this merged. @tianon can you update it for the 3.5 variants?

[tianon]

Updated! 👍