bakefile: try to keep defaults in the Dockerfile

[thaJeztah]

This is an attempt to keep defaults in the Dockerfile, without having to re-define them in the bakefile. Before this patch, build-args would always be set, but either with a default (specified in the bakefile) or an empty string.

When setting a build-arg to an empty string, the default from the Dockerfile is ignored (and a literal empty string used instead).

Ideally, bakefiles would have some option for this, but I don't think there currently is, but thinking something along the lines of;

• a list of build-arg names to consider (taking them from the environment, if present)
• possibly changing args to a list of strings instead of a map (so that key can be set without =value), although this would still require some mangling of the values (must be set without = to take environment variables).
• maybe other options?

Other things I looked at (but I'm very much not familiar with the HCL syntax);

• custom conditions (this could reduce the boilerplating somevar != "" see https://developer.hashicorp.com/terraform/language/expressions/custom-conditions
• local values; trying to use local (or locals) didn't work; trying to reference them through local.variable_name caused an error (but so did referencing variables through var.variable_name), perhaps it's just not supported by bake (again, I'm very unfamiliar with HCL so maybe I just did it wrong); https://developer.hashicorp.com/terraform/language/values/locals The HCL docs seem to mention them, so perhaps it's just not supported by bake; https://github.com/hashicorp/hcl/blob/27df1ec5fcbc64f6799707e98c7d0433b05ef140/guide/language_design.rst#blocks-vs-object-values

I also looked at the current bakefile, and was a bit confused how merging of maps happens (looks like inherits does an implicit merge of maps).

Either way, with this patch:

The default is taken from the Dockerfile;

docker buildx bake --set binary.platform=darwin/amd64,darwin/arm64 binary
...
=> [linux/arm64/v8 internal] load metadata for docker.io/library/golang:1.19.3-alpine   2.4s
...

But can still be overridden;

GO_VERSION=1.19.2 docker buildx bake --set binary.platform=darwin/amd64,darwin/arm64 binary
...
=> [linux/arm64/v8 internal] load metadata for docker.io/library/golang:1.19.2-alpine   2.4s
...

- A picture of a cute animal (not mandatory but encouraged)

[codecov-commenter]

Codecov Report

Merging #3898 (2f6e3c9) into master (64c8976) will not change coverage.
The diff coverage is n/a.

Additional details and impacted files

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##           master    #3898   +/-   ##
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  Coverage   59.22%   59.22%           
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  Files         287      287           
  Lines       24712    24712           
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  Hits        14635    14635           
  Misses       9192     9192           
  Partials      885      885           

[thaJeztah]

Github having issues?

#19 https://github.com/theupdateframework/notary/releases/download/v0.6.1/notary-Linux-amd64
#19 ERROR: invalid response status 503

#20 [build-base-alpine 2/4] COPY --from=xx / /
#20 CANCELED
------
 > https://github.com/theupdateframework/notary/releases/download/v0.6.1/notary-Linux-amd64:
------
ERROR: failed to solve: failed to load cache key: invalid response status 503
make: *** [docker.Makefile:124: build-e2e-image] Error 1
Error: Process completed with exit code 2.

[crazy-max]

docker/buildx#1449 should fix this issue so we avoid sending empty args

[thaJeztah]

docker/buildx#1449 should fix this issue so we avoid sending empty args

Ah, nice! Does it allow distinguishing between VAR="" / VAR= (override with empty string) and VAR ?

[crazy-max]

Yes if you put VAR="", it will override with an empty string but not VAR=null.