Follow-up to #11, which landed the trigger-form handling but not the rest.
Three things still stood between FORCE_REBUILD and an actual republish.
Declare FORCE_REBUILD as a top-level variable so the UI offers it as a
dropdown on a manual run, rather than requiring a hand-typed key.
Test the value rather than emptiness. A declared variable substitutes its
default, so the `[ -n ... ]` check from #11 would have seen the literal
string "false" and rebuilt every platform on every cron run.
Set overwrite: true on the upload. plugin-s3 skips an object that already
exists when overwrite is false, logging "File already exists, skipping
upload" and still exiting green, so a forced rebuild would have recompiled
every package and published none of them. Normal runs are unaffected, since
dedup means only not-yet-published versions are ever built.
Also drop the Net debug step, the commented-out Pull upstream step and a
stray `# docker info`. The MTU workaround the debug step was diagnosing
stays, as does the commented-out alpine-320 matrix entry, which records a
deliberately disabled platform rather than dead scaffolding.
