Instructions on how to use plugin from local source code
**Describe Your Problem:**
I'm trying to test a plugin that I have forked and modified locally.
I'd obviously like to include that in the docker image. I'm not totally familiar with npm and especially how that's handled in homebridge with plugins, so I hope this isn't something obvious.

Are there instructions as to what the recommended procedure is?

```dockerfile
FROM oznu/homebridge:latest

ADD . /hmip/  # add local clone of https://github.com/thkl/homebridge-homematic
RUN npm install /hmip && \
    npm list
```
I also played with some alternative dockerfiles using `npm link` or a `npm install -g /hmip`.
In none of my tests I managed to get my modified code running in homebridge.

I'm guessing that maybe the startup procedure that installs plugins overwrites the locally installed version?

**Logs:**
<!-- Support requests that do not contain logs may be closed without warning. -->

```
Show the Docker container logs here
```

**Docker Start Command:**
I use docker-compose:


```yaml
version: "3"
services:
  homebridge:
    build:
      context: .
    image: my-homebridge:latest
    #image: oznu/homebridge
    command: ["-D", "-I"]
    restart: always
    environment:
      - TZ=Europe/Berlin
      - PGID=1000
      - PUID=1000
      - HOMEBRIDGE_CONFIG_UI=1
      - HOMEBRIDGE_CONFIG_UI_PORT=8080
      - HOMEBRIDGE_DEBUG=1
      - DEBUG=alexa*
    ports:
      - ...
```

**Homebridge Config:**

```json
Show your homebridge config.json here
```

**Screenshots:**
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**Environment:**

* **Host Operating System**: <!-- Raspbian / Ubuntu / Debian / Windows / macOS etc. -->
* **Docker Version**: <!-- docker -->
* **Image Tag / Type**: latest

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