test: Add Docker export test with mock builder implementation

[ImBIOS]

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It introduces a new file export_test.go with unit tests for the export functionality. The tests use a mock implementation of the builder.Impl interface called MockBuilder.

The commit also includes an update to the go.mod file, adding the github.com/lib/pq and github.com/stretchr/testify dependencies.

Lastly, the go.sum file is updated with the checksum for the github.com/stretchr/objx v0.5.0 dependency.

Related:

• #1316

[encore-cla[bot]]

All committers have signed the CLA.

[eandre]

I believe the original root cause has been fixed, and we're about to land some large improvements to the docker export functionality, so closing this as it's about to be superceded.