Add Documentations about DEV container (#7768)

Signed-off-by: xiaofan-luan <xiaofan.luan@zilliz.com>

build/README.md

@@ -73,6 +73,57 @@ export OS_NAME=centos7
 build/builder.sh make
 ```
 
+## Dev Containers
+Users can also get into the dev containers for development.
+
+Enter root path of Milvus project on your host machine, execute the following commands:
+
+```shell
+$ ./scripts/devcontainer.sh up        # start Dev container
+
+Creating network "milvus-distributed_milvus" with the default driver
+Creating milvus_jaeger_1 ... done
+Creating milvus_minio_1  ... done
+Creating milvus_pulsar_1 ... done
+Creating milvus_etcd_1   ... done
+Creating milvus_ubuntu_1 ... done
+```
+
+Check running state of Dev Container:
+
+```shell
+docker ps
+
+CONTAINER ID        IMAGE                                                               COMMAND                  CREATED             STATUS                             PORTS                                                                NAMES
+8835ee913953        quay.io/coreos/etcd:v3.4.13                                         "etcd -advertise-cli…"   30 seconds ago      Up 29 seconds                      2379-2380/tcp                                                        milvus-distributed_etcd_1
+3bd7912f5e98        milvusdb/milvus-distributed-dev:amd64-ubuntu18.04-20201209-104246   "autouseradd --user …"   30 seconds ago      Up 29 seconds                      22/tcp, 7777/tcp                                                     milvus-distributed_ubuntu_1
+9fa983091d3d        apachepulsar/pulsar:2.6.1                                           "bin/pulsar standalo…"   30 seconds ago      Up 29 seconds                                                                                           milvus-distributed_pulsar_1
+80687651517b        jaegertracing/all-in-one:latest                                     "/go/bin/all-in-one-…"   30 seconds ago      Up 29 seconds                      5775/udp, 5778/tcp, 14250/tcp, 14268/tcp, 6831-6832/udp, 16686/tcp   milvus-distributed_jaeger_1
+22190b591d74        minio/minio:RELEASE.2020-12-03T00-03-10Z     
+```
+
+3bd7912f5e98 is the docker of milvus dev, other containers are used as unit test dependencies. you can run compile and unit inside the container, enter it:
+
+```shell
+docker exec -ti 3bd7912f5e98 bash
+```
+
+Compile the project and run unit test, see details at the DEVELOPMENT.md
+
+```shell
+make all
+```
+
+```shell
+make unittest
+```
+
+Stop Dev Container 
+
+```shell
+./scripts/devcontainer.sh down # close Dev container
+```
+
 ## E2E Tests
 
 Milvus uses Python SDK to write test cases to verify the correctness of Milvus functions. Before run E2E tests, you need a running Milvus:
@@ -103,3 +154,7 @@ docker-compose run --rm pytest /bin/bash -c "pytest --host ${MILVUS_SERVICE_IP}"
 The scripts directly under [`build/`](.) are used to build and test. They will ensure that the `builder` Docker image is built (based on [`build/docker/builder`] ) and then execute the appropriate command in that container. These scripts will both ensure that the right data is cached from run to run for incremental builds and will copy the results back out of the container. You can specify a different registry/name for `builder` by setting `IMAGE_REPO` which defaults to  `milvusdb`.
 
 The `builder.sh` is execute by first creating a “docker volume“ directory in `.docker/`. The `.docker/` directory is used to cache the third-party package and compiler cache data. It speeds up recompilation by caching previous compilations and detecting when the same compilation is being done again.
+
+## CROSS DEBUGGING
+Need detailed documentation about how to cross debugging between host machine IDE and Dev containers, TODO.
+