Add support for automatic tagging (:1, :1.2, :1.2.3, etc) for semantic versions

[kylelemons]

Enabling the feature

This feature is off by default. To enable it, either:

• PLUGIN_AUTO_TAG for drone plugins
• --auto_tag for command-line use

This changes the way tags (both --tags and PLUGIN_TAGS) are interpreted. Now, instead of saying 1,1.2,1.2.3,latest you can say v1.2.3,latest

This mostly follows semver.org, except that it also swaps underscores (which are not permitted) for hyphens, as otherwise it will likely be confusing when you try v1.2.3+linux_arm and your semver tagging breaks.

Semantics

This parses all tags as semantic versions (after performing a swap of _ for - as required by semver.org). If they parse correctly, the following logic happens:

• If the semver is pre-release, it is emitted directly (e.g. v1.2.3-alpha1 tags 1.2.3-alpha1)
• If the semver has build info, it is appended to all tags (e.g. v1.2.3+linux tags 1+linux, 1.2+linux, etc)
• If it is not a valid semver, nothing changes

Alternatives

Instead of interpreting PLUGIN_TAGS differently, we could make PLUGIN_AUTO_TAGS a list that would be required to parse correctly as semantic versions, which might avoid some classes of error. That would make invocation look like

-e PLUGIN_TAGS=latest
-e PLUGIN_AUTO_TAGS=$DRONE_TAG

This probably works fine for most drone workflows, though it will break workflows where e.g. someone tags a build for_testing or something along those lines, so I did not go with this approach initially.

[shubham149]

Sorry for late review. I missed the notification for this pr.

[kylelemons]

Sorry for late review.

Oof, sorry for the even later reply. This fell so far down my TODO list that I lost it :(

We've been using this for awhile now so I'm pretty confident that it works -- the main "gotcha" is that if you forget the "v" you will only get the numeric tag (e.g. if you tag "1.2.3" instead of "v1.2.3" you won't get ":1" or ":1.2"). I think it's especially easy to forget the v since docker tags don't have them.

So, I have also included (as a single commit, in case you'd like me to revert this piece) a change that will recognize them as well.

[shubham149]

LGTM. We can merge after the build check passes.