Switch to Alpine 3.8 as base image

[bertinatto]

Fixes #59.

Image size decreases considerably:

quay.io/bertinatto/ebs-csi-driver           testing            b73128d25c5d   2 minutes ago    30.2MB

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[coveralls]

Pull Request Test Coverage Report for Build 70

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 49.058%

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Totals
Change from base Build 69:	0.0%
Covered Lines:	521
Relevant Lines:	1062

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💛 - Coveralls

[bertinatto]

@leakingtapan, what do you think about this?

I don't have any problems using any other base image, like Amazon Linux. However, Alpine seemed to be the lighter alternative.

[leakingtapan]

Just realize one existing issue, could you switch RUN apk add... with COPY so that docker build doesn't generate new mid layer each time we make changes to driver?

[bertinatto]

I'm not sure if I understand. We need to install e2fsprogs and ca-certificates, and the way to do that in Alpine is with pkg add. Not sure if we can do that with COPY.

[leakingtapan]

I mean switching the line order to be like:

RUN apk add --no-cache ca-certificates e2fsprogs
COPY --from=builder /go/src/github.com/kubernetes-sigs/aws-ebs-csi-driver/bin/aws-ebs-csi-driver /bin/aws-ebs-csi-driver

Because each line in docker file generates a new layer of docker image, we could avoid keep generating the apk add layer if it is before COPY line

[bertinatto]

Gotcha... Done

[leakingtapan]

/lgtm