[bug] failed to make build-cli to build pouch cli

[allencloud]

Ⅰ. Issue Description

I found that recently the pouchrobot failed to generate cli docs automatically.

Please see the failure log. It seems to be related with buildkit code which is introduced in #2779.

root@7a95dc252894:/go/src/github.com/alibaba/pouch# make build-cli
build-cli: bin/pouch
# github.com/alibaba/pouch/vendor/github.com/moby/buildkit/util/apicaps
vendor/github.com/moby/buildkit/util/apicaps/caps.go:147:8: undefined: strings.Builder
Makefile:125: recipe for target 'build-cli' failed
make: *** [build-cli] Error 2
root@7a95dc252894:/go/src/github.com/alibaba/pouch# git log
commit 5c253f937da2bfb4cd030d81800ecf1145805896
Author: Rudy Zhang <rudyflyzhang@gmail.com>
Date:   Mon May 13 14:22:22 2019 +0800

    remove useless break statement

    Signed-off-by: Rudy Zhang <rudyflyzhang@gmail.com>

commit 2596dc7bc81e7fff6ffa481c0decb53ddb2b036d

Ⅱ. Describe what happened

make build-cli failed.

Ⅲ. Describe what you expected to happen

make build-cli works

Ⅳ. How to reproduce it (as minimally and precisely as possible)

see the command line display above.

Ⅴ. Anything else we need to know?

none

Ⅵ. Environment:

• pouch version (use pouch version):
• OS (e.g. from /etc/os-release):
• Kernel (e.g. uname -a):
• Install tools:
• Others:

[fuweid]

@allencloud the strings.Builder is introduced in go1.10 https://golang.org/doc/go1.10#strings . Please update your golang version. Thanks