HTTP_PROXY doesn't update when minikube start with different proxy

[linbo]

Is this a BUG REPORT or FEATURE REQUEST? (choose one):
BUG REPORT

Please provide the following details:

Environment:

Minikube version (use minikube version): v0.30.0

• OS (e.g. from /etc/os-release): Win10
• VM Driver (e.g. cat ~/.minikube/machines/minikube/config.json | grep DriverName): virtualbox
• ISO version (e.g. cat ~/.minikube/machines/minikube/config.json | grep -i ISO or minikube ssh cat /etc/VERSION): minikube-v0.30.0.iso
• Install tools: choco
• Others:

What happened:

1. minikube start  --docker-env=HTTP_PROXY=http://ip1:port1 --docker-env=HTTPS_PROXY=https://ip1:port1
2. minikube stop
3. minikube start  --docker-env=HTTP_PROXY=http://ip2:port2 --docker-env=HTTPS_PROXY=https://ip2:port2

After finish those command, docker http proxy still remain ip1:port1, not ip2:port2

What you expected to happen:
docker http proxy should reset to ip2:port2

How to reproduce it (as minimally and precisely as possible):

1. minikube start  --docker-env=HTTP_PROXY=http://ip1:port1 --docker-env=HTTPS_PROXY=https://ip1:port1
2. minikube stop
3. minikube start  --docker-env=HTTP_PROXY=http://ip2:port2 --docker-env=HTTPS_PROXY=https://ip2:port2

Output of minikube logs (if applicable):

Anything else do we need to know: