chore: avoid unnecessary image pulls while testing

[johnstcn]

While running integration tests:

        integration_test.go:919: "error: GET https://index.docker.io/v2/library/alpine/manifests/latest: TOOMANYREQUESTS: You have reached your pull rate limit. You may increase the limit by authenticating and upgrading: https://www.docker.com/increase-rate-limit"
        integration_test.go:444: 
                Error Trace:    /workspaces/envbuilder/integration/integration_test.go:444
                Error:          Received unexpected error:
                                error: GET https://index.docker.io/v2/library/alpine/manifests/latest: TOOMANYREQUESTS: You have reached your pull rate limit. You may increase the limit by authenticating and upgrading: https://www.docker.com/increase-rate-limit
                Test:           TestBuildFailsFallback/NoImageOrDockerfile

We should stand up a test registry server and build a local test image so that we don't end up spamming Docker Hub.

[johnstcn]

Fixed by #138