chore: Add Docker image tests

[nieomylnieja]

Motivation

Currently we're not really testing the Dockerfile image directly nor the built images.
In our bats tests we're just using the Dockerfile to build the sloctl binary and copy it to bats container.
Since running dind (Docker in Docker) images has proven tricky to me I've decided to write this basic tests in Go instead.
The tests should only check basic interactions and not cover the larger spectrum that bats tests cover.

Summary

Updated workflows for e2e tests as well as release workflow.

Testing

Run:

SLOCTL_CLIENT_ID=<ID> SLOCTL_CLIENT_SECRET=*** SLOCTL_OKTA_ORG_URL=<ORG_URL> SLOCTL_OKTA_AUTH_SERVER=<SERVER> SLOCTL_E2E_DOCKER_TEST_IMAGE=nobl9/sloctl make test/go/e2e

Note that SLOCTL_E2E_DOCKER_TEST_IMAGE is optional, if not provided the image will be built withing the test code.

Example dispatch runs:

• https://github.com/nobl9/sloctl/actions/runs/9697641809
• https://github.com/nobl9/sloctl/actions/runs/9697743409