default semver constraint for empty string

[zywillc]

Closes #556

This PR is to fix 2 issues:

1. current tags filtering behavior will skip semver constraint for prereleases versions (although they are valid semver versions) when there is no explicit constraint parsed from image tags (i.e. empty string) which is inconsistent with the document description.
   https://github.com/argoproj-labs/argocd-image-updater/blob/master/pkg/image/version.go#L135-L147

2. some complex prerelease constraint rules are actually not supported untilsemver > 3.2.0

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 74.85%. Comparing base (4f21ade) to head (5625cdb).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #552      +/-   ##
==========================================
+ Coverage   74.82%   74.85%   +0.03%     
==========================================
  Files          31       31              
  Lines        3912     3917       +5     
==========================================
+ Hits         2927     2932       +5     
  Misses        850      850              
  Partials      135      135              

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[jannfis]

Thanks for your contribution and apologies that it took so long to come back to this PR.

Would you mind to rebase this PR (some of the lint changes have already been merged through another PR), and to supply a unit test for the * case?

Thanks a lot!

[chengfang]

@zywillc I've rebased and the result is 1 file change in version.go. Can you review it, and maybe add some unit tests?

argocd-image-updater/pkg/image/version_test.go

Line 30 in e193d9d

func Test_LatestVersion(t *testing.T) {

seems to be a good place for such tests.

[chengfang]

I did some manual testing with the current master, WITHOUT changes in this PR, and here are my findings:

• with no constraint ("" constraint), image updater picks the latest version including pre-release versions. If a new pre-release version exists, it will be selected as the latest one instead of any older stable version.

dist/argocd-image-updater test quay.io/cfang/argocd-operator --update-strategy semver
INFO[0002] latest image according to constraint is quay.io/cfang/argocd-operator:1.2.0-rc1

• with constraint "*", image updater chooses from stable versions only.

dist/argocd-image-updater test quay.io/cfang/argocd-operator --update-strategy semver --semver-constraint="*"
INFO[0004] latest image according to constraint is quay.io/cfang/argocd-operator:1.1.0

• with constraint "x.x.x-0", image updater chooses from both stable and pre-releases versions. If a new pre-release version exists, it will be selected as the latest one instead of any older stable version.

dist/argocd-image-updater test quay.io/cfang/argocd-operator --update-strategy semver --semver-constraint="x.x.x-0"
INFO[0001] latest image according to constraint is quay.io/cfang/argocd-operator:1.2.0-rc1

I think the current behavior already matches the doc and your expectation, and no changes are needed.