build/Dockerfile defines ARG before FROM

[nresare]

When looking at build/Dockerfile it seems it should be configurable which binary it will use, but due to a peculiarity of the Dockerfile format, the value of BINARY will only every be 'flipt'.

The relevant detail, in the Dockerfile reference:

An ARG declared before a FROM is outside of a build stage, so it can't be used in any instruction after a FROM.

It turns out that it is not quite true, it seems from experimentation that the default value of BINARY will still be accessible, it is just not configurable at build time. Moreover, any docker build invocation will print the following warning:

 1 warning found (use docker --debug to expand):
 - UndefinedVar: Usage of undefined variable '$BINARY' (line 14)

I would suggest either removing the configurability of BINARY by simply hard coding it in the COPY line, or moving the ARG below the FROM line

[dosubot]

Hi @nresare! I'm Dosu and I’m helping the flipt team.

You're right—defining ARG BINARY before FROM in the Dockerfile means its value isn't configurable at build time, and it always defaults to flipt. This also triggers the warning about the undefined variable in the COPY $BINARY / line. To make BINARY configurable, move the ARG BINARY=flipt line below the FROM instruction. If configurability isn't needed, just hardcode the binary name in the COPY instruction. There’s no repository-specific reason for the current setup, so either approach would resolve the warning and clarify intent in the Dockerfile.
See the Dockerfile here.

To reply, just mention @dosu.

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