The environment variable `BUILDX_BAKE_FILE` (and optional variable
`BUILDX_BAKE_FILE_SEPARATOR`) can be used to specify one or more bake
files (similar to `compose`). This is mutually exclusive with`--file`
(which takes precedence).
This is done very early to ensure the values are treated just like
`--file`, e.g., participate in telemetry. This includes leaving
relative paths as-is, which deviates from `compose` (which makes them
absolute).
Signed-off-by: Roberto Villarreal <rrjjvv@yahoo.com>