feat(discord/build): one code block per tag in copy reply

[barnabasbusa]

Summary

• "Copy tag" button now returns an ephemeral reply with each image tag in its own triple-backtick block, so Discord's per-block copy icon grabs exactly one tag — no duplicated link+block, no risk of copying every tag at once.
• Button pluralizes to "Copy tags" when a build produces more than one image.
• "Show another image…" dropdown removed (redundant — all tags are already in the reply). dockerImage.Label() dropped with it.

Test plan

• Trigger a single-image build, click Copy tag in the DM, verify the ephemeral reply is one code block and Discord's copy icon yields the tag.
• Trigger a multi-image build (variants), verify button label reads Copy tags, reply contains one code block per image, and each code block copies its own tag independently.
• Confirm no dropdown is rendered alongside the button.
• Stale-cache path: restart the bot or wait past the completion TTL, click the button, verify the "no longer in cache" ephemeral still fires.