"Empty continuation lines will become errors"… how should I comment my Dockerfile now?

Question

Consider the following Dockerfile:

FROM alpine:edge  EXPOSE \ # web portal 8080 \ # backdoor 8081 

Built like so:

docker build . 

We observe such output:

Sending build context to Docker daemon  17.1TB Step 1/2 : FROM alpine:edge  ---> 7463224280b0 Step 2/2 : EXPOSE 8080 8081  ---> Using cache  ---> 7953f8df04d9 [WARNING]: Empty continuation line found in:     EXPOSE 8080 8081 [WARNING]: Empty continuation lines will become errors in a future release. Successfully built 7953f8df04d9 

So, given that it'll soon become illegal to put comments in the middle of a multi-line section: what's the new recommended way to comment multi-line commands?

This is particularly important for RUN commands, since we are encouraged to reduce image layers by &&ing commands together.

---

Not sure exactly when this was introduced, but I'm currently experiencing this in version:

🍔 docker --version Docker version 17.07.0-ce, build 8784753 

I'm using Docker's edge release stream, so maybe this will not yet look familiar if you are using Docker stable.

Answer 1

17.07.0-ce started to warn on empty continuation lines. However, it incorrectly treated comment-only lines as empty. This is fixed in moby#35004, and being included in the 17.10.0-ce.

Answer 2

On top of what others have said above (the error might be related to comments inside continuation blocks and/or windows cr/lf characters = use dos2unix), this message can also show up when your last command ends with a backslash \ character. For example, if you have this:

RUN apt-get update \     && apt-get upgrade \     && apt-get -y install build-essential curl gnupg libfontconfig ca-certificates bzip2 \     && curl -sL https://deb.nodesource.com/setup_16.x  | bash - \     && apt-get -y install nodejs \     && apt-get clean \     && rm -rf /tmp/* /var/lib/apt/lists/* \ 

Notice the last \ at the end. This will get you the same error:

docker [WARNING]: Empty continuation line found in:

So, just remove that last \ and you're all set.