Allow mounting in DOCKER_CONFIG again

[johnstcn]

Motivation

In #336 we temporarily override DOCKER_CONFIG to set the config specified by ENVBUILDER_DOCKER_CONFIG_BASE64.

This breaks some existing users who were mounting in a docker config and specifying the DOCKER_CONFIG environment variable in their image.

Proposed solution

Only override DOCKER_CONFIG if ENVBUILDER_DOCKER_CONFIG_BASE64 is set. Otherwise, leave it as-is.

[mafredri]

Only override DOCKER_CONFIG if ENVBUILDER_DOCKER_CONFIG_BASE64 is set. Otherwise, leave it as-is.

@johnstcn It looks like we do this already:

envbuilder/envbuilder.go

Lines 1633 to 1635 in 08bdb8d

	if dockerConfigBase64 == "" {
	return noop, nil
	}

Is this a problem with the latest version of envbuilder or is the customer looking for some other behavior than mentioned?

If the expectation is to never overwrite DOCKER_CONFIG, then setting both DOCKER_CONFIG and ENVBUILDER_DOCKER_CONFIG_BASE64 should probably result in an error. We can't know which should take precedence.

[johnstcn]

Ah, good point. The difference in behaviour was pointed out between v1.0.0-rc.6 and v1.0.0-rc.7.

[mafredri]

Ok, so in v1.0.0-rc.7 the behavior was changed so that if you don't set base64, DOCKER_CONFIG doesn't get set either. Meaning if you define DOCKER_CONFIG, that's the one that will be used. However, if you were relying on envbuilder to search for a Docker config at /.envbuilder/config.json, then v1.0.0-rc.7 would break your use-case. Could this be what's happening?

[johnstcn]

Confirmed that this is what's happening.