bake: add file-relative path opt-in

Signed-off-by: CrazyMax <1951866+crazy-max@users.noreply.github.com>

docs/bake-reference.md

@@ -416,6 +416,10 @@ target "app" {
 ```
 
 This resolves to the current working directory (`"."`) by default.
+Set `BUILDX_BAKE_FILE_RELATIVE_PATHS=1` to resolve local directory paths in
+`target.context` and `target.contexts` relative to the directory of the first
+Bake file. Use `cwd://` for paths that should remain relative to the current
+working directory when this opt-in is enabled.
 
 ```console
 $ docker buildx bake --print -f - <<< 'target "default" {}'

docs/reference/buildx_bake.md

@@ -153,6 +153,12 @@ This is mutually exclusive with `-f` / `--file`; if both are specified, the envi
 Multiple definitions can be specified by separating them with the system's path separator
 (typically `;` on Windows and `:` elsewhere), but can be changed with `BUILDX_BAKE_PATH_SEPARATOR`.
 
+By default, local directory build contexts in Bake files are resolved from the
+current working directory. To opt in to resolving local directory build contexts
+from the directory of the first Bake file, set
+`BUILDX_BAKE_FILE_RELATIVE_PATHS=1`. Use the `cwd://` prefix for paths that
+should remain relative to the current working directory.
+
 You can pass the names of the targets to build, to build only specific target(s).
 The following example builds the `db` and `webapp-release` targets that are
 defined in the `docker-bake.dev.hcl` file: