fix docker library compatibility

[bsdlp]

Description

samalba/dockerclient depends on a feature that docker/docker no longer supports, causing:

jchen@localhost-(main|✚2…)> go mod tidy
go: finding module for package github.com/samalba/dockerclient
go: found github.com/samalba/dockerclient in github.com/samalba/dockerclient v0.0.0-20160531175551-a30362618471
go: finding module for package github.com/docker/docker/pkg/jsonlog
go: github.com/voltaire/render-controller/renderer/internal/linode imports
        github.com/docker/machine/libmachine imports
        github.com/docker/machine/libmachine/provision imports
        github.com/samalba/dockerclient tested by
        github.com/samalba/dockerclient.test imports
        github.com/docker/docker/pkg/jsonlog: package provided by github.com/docker/docker at latest version v1.13.1 but not at required version v17.12.0-ce-rc1.0.20200916142827-bd33bbf0497b+incompatible

this patch removes the no longer maintained samalba/dockerclient in favor of docker/docker/client.

this patch also updates the go version in Dockerfile to 1.15 in order to fix:

go build \
-o /go/src/github.com/docker/machine/bin/docker-machine \
 -tags "" \
-ldflags "-X `go list ./version`.GitCommit=`git rev-parse --short HEAD 2>/dev/null` -w -s"  ./cmd/docker-machine
# github.com/docker/machine/vendor/github.com/containerd/containerd/errdefs
vendor/github.com/containerd/containerd/errdefs/errors.go:54:9: undefined: "github.com/docker/machine/vendor/github.com/pkg/errors".Is
vendor/github.com/containerd/containerd/errdefs/errors.go:59:9: undefined: "github.com/docker/machine/vendor/github.com/pkg/errors".Is
vendor/github.com/containerd/containerd/errdefs/errors.go:65:9: undefined: "github.com/docker/machine/vendor/github.com/pkg/errors".Is
vendor/github.com/containerd/containerd/errdefs/errors.go:71:9: undefined: "github.com/docker/machine/vendor/github.com/pkg/errors".Is
vendor/github.com/containerd/containerd/errdefs/errors.go:76:9: undefined: "github.com/docker/machine/vendor/github.com/pkg/errors".Is
vendor/github.com/containerd/containerd/errdefs/errors.go:81:9: undefined: "github.com/docker/machine/vendor/github.com/pkg/errors".Is
vendor/github.com/containerd/containerd/errdefs/errors.go:86:9: undefined: "github.com/docker/machine/vendor/github.com/pkg/errors".Is
vendor/github.com/containerd/containerd/errdefs/errors.go:92:9: undefined: "github.com/docker/machine/vendor/github.com/pkg/errors".Is
make: *** [mk/build.mk:32: /go/src/github.com/docker/machine/bin/docker-machine] Error 2
make: *** [Makefile:15: build] Error 2

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