add context input to replace workdir and source inputs

[crazy-max]

• fixes Workdir does not work or is not documented properly #294
• fixes Bad context on version 6.x.x #301
• fixes Uncertain behavior of local source #352

This PR introduces a new context input that replaces both workdir and source inputs. It resolves long-standing confusion around how bake-action determines what directory or Git reference to use when running bake.

The workdir input was originally added in #76 to support running Bake from a subdirectory inside the GitHub workspace.

Later, Git context support was added in #181 introducing a new source input. This created overlap and ambiguity between:

• workdir (local path inside workspace)
• source (Git context but can be skipped with .)

This duality has consistently caused confusion for users, as highlighted in multiple issues and discussions. Users also pointed out that naming was inconsistent with build-push-action, where the input is called context.

This unified context input aligns with build-push-action. If a local dir is detected it will use it as working directory to run Bake otherwise it will use the Git context like it does for build-push-action.

cc @ModerNews @polarathene @Clashsoft