fix(dockerfile) manage ENV legacy case without an equal sign

[dduportal]

Fix #390

This PR add the support for ENV Dockerfile instruction without equal sign (which is considered "legacy" and not recommended by Docker as per https://docs.docker.com/engine/reference/builder/#env)

⚠️ Alternative syntax
The ENV instruction also allows an alternative syntax ENV , omitting the =. For example:

   ENV MY_VAR my-value

This syntax does not allow for multiple environment-variables to be set in a single ENV instruction, and can be confusing. For example, the following sets a single environment variable (ONE) with value "TWO= THREE=world":

   ENV ONE TWO= THREE=world

The alternative syntax is supported for backward compatibility, but discouraged for the reasons outlined above, and may be removed in a future release.

The behavior in these cases is to rewrite with the new "recommended" syntax which inserts an equal sign.

For instance in the reproduction case from #390:

updateReleaseInDockerfile
-------------------------
**Dry Run enabled**

🐋 On (Docker)file ".tmp/Dockerfile":

⚠ The line #3, matched by the keyword "ENV" and the matcher "WINDOWS_UPDATE_VERSION",
is changed from "ENV WINDOWS_UPDATE_VERSION 0.10.1" to "ENV WINDOWS_UPDATE_VERSION=v0.14.0".

Test

To test this pull request, you can run the following commands:

make test-short
go build -o ./dist/updatecli && ./dist/updatecli diff --config examples/updatecli.generic/dockerfile/
make test-e2e

Additional Information

[lemeurherve]

(out of scope?) I would add a test with more than one variable defined by ENV on a same line.

[dduportal]

(out of scope?) I would add a test with more than one variable defined by ENV on a same line.

Yep, that would be a separated feature request :)

But I would not (personnaly, don't know for others) spend time on such feature request as even if supported by Dockerfile, multiline ENV are really a bad pattern and make no sense since buildkit is out of the box (the only reason to group ENV in a multiline is to shrink the layer that it was generating before Docker 19.03).

However, if you feel like it's a good idea, would you be willing to send a PR that adds this test + show a warning to end user to remind that it is not supported by updatecli (with a if contains ` at the end after trimming or something like this)?