Update install.md

## Compile Milvus on Docker

With this dockerfile, you should be able to compile CPU-only or GPU-enabled Milvus on any Linux platform that run Docker. To build a GPU-enabled Milvus, you neeed to install [NVIDIA Docker](https://github.com/NVIDIA/nvidia-docker/) first.

With this dockerfile, you should be able to compile Milvus on any Linux platform that run Docker. To build a GPU supported Milvus, you neeed to install [NVIDIA Docker](https://github.com/NVIDIA/nvidia-docker/) first.

### Step 1 Pull Milvus Docker images

#### Step 1 Pull Milvus Docker images

Pull CPU-only image:

```shell

$ Docker pull milvusdb/milvus-gpu-build-env:v0.6.0-ubuntu18.04

```

### Step 2 Start the Docker container

#### Step 2 Start the Docker container

Start a CPU-only container:

```shell

$ Docker exec -it [container_id] bash

```

### Step 3 Download Milvus source code and compile it in the container

#### Step 3 Download Milvus source code

Download Milvus source code:

$ unzip ./0.6.0.zip

```

The source code is extracted into a folder called `milvus-0.6.0`. 

The source code is extracted into a folder called `milvus-0.6.0`. To enter its core directory:

```shell

$ cd ./milvus-0.6.0/core

```

#### Step 4 Compile Milvus in the container

If you are using a CPU-only image, compile it like this:

```shell

$ ./build.sh -t Release

```

For GPU-enabled image, you need to add a `-g` parameter:

If you are using a GPU-enabled image, you need to add a `-g` parameter:

```shell

$ ./build.sh -g -t Release

```