[chore] [cmd/builder] update README.md (#12821)

Fix
open-telemetry/opentelemetry-collector-releases#911

<!--Ex. Fixing a bug - Describe the bug and how this fixes the issue.
Ex. Adding a feature - Explain what this achieves.-->
#### Description
Fixes documentation issue where ocb does not respect a default config
file (needs to be specified via `--config`
<!-- Issue number if applicable -->
#### Link to tracking issue
Fixes
[#911](open-telemetry/opentelemetry-collector-releases#911)

<!--Describe what testing was performed and which tests were added.-->
#### Testing
None, doc only
<!--Describe the documentation added.-->
#### Documentation
README.md
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Author: jackgopack4

cmd/builder/README.md

@@ -77,9 +77,11 @@ The included builder configuration file/manifest should be replaced by mounting
 Assuming you are running this image in your working directory, have a `builder-config.yaml` file located in this folder, the `dist.output_path` item inside your `builder-config.yaml` is set to `./otelcol-dev`, and you wish to output the binary/go module files to a folder named `output`, the command would look as follows:
 
 ```
-docker run -v "$(pwd)/builder-config.yaml:/build/builder-config.yaml" -v "$(pwd)/output:/build/otelcol-dev" otel/opentelemetry-collector-builder:latest
+docker run -v "$(pwd)/builder-config.yaml:/build/builder-config.yaml" -v "$(pwd)/output:/build/otelcol-dev" otel/opentelemetry-collector-builder:latest --config=/build/builder-config.yaml
 ```
 
+Please note that a `--config` flag must be passed to specify your custom manifest.yaml/builder-config.yaml file regardless of where you mount it inside the container, otherwise a default config is used that cannot be changed.
+
 Additional arguments may be passed to ocb on the command line as specified below, but if you wish to do this, you must make sure to pass the `--config` argument, as this is specified as an additional `CMD`, not an entrypoint. 
 
 ### Official release binaries