Honor stdout and stderr false in Container#exec

[tas50]

Container#exec resolved its stream options with

stdout = options.delete(:stdout) || !detach
stderr = options.delete(:stderr) || !detach

An explicit false is falsy, so it fell through to !detach and came back as true. Neither stream could be switched off.

The README documented container.exec(["date"], stderr: false) as the way to capture only stdout. It never did:

stderr: false  ->  stdout=["O\n"] stderr=["E\n"]   # not suppressed

The fix

Test for the key rather than its truthiness, keeping !detach as the default when the caller says nothing:

stdout = options.key?(:stdout) ? options.delete(:stdout) : !detach
stderr = options.key?(:stderr) ? options.delete(:stderr) : !detach

Verification

Two specs, both confirmed failing before the change:

expected `["err\n"].empty?` to be truthy, got false    # stderr: false
expected `["out\n"].empty?` to be truthy, got false    # stdout: false

Both pass after. The existing #exec examples — default behaviour, detach: true, block streaming, tty — are unchanged, and exec_spec is 11 examples / 0 failures.

Compatibility

Passing detach:, or passing nothing, behaves exactly as before. Only an explicit stdout: false / stderr: false changes, and that is the documented behaviour finally working rather than a contract anyone could have depended on.

Note on CI

#exec when stdin object is passed fails on this branch, but is pre-existing on master — I confirmed master alone reports 5 examples, 1 failure for that same spec. It is the hijack/attach issue and only reproduces on Docker Desktop; it passes on the Linux runners. CI will also stay red here until #605 lands.