Update to golang 1.18

[stephen-soltesz]

This change updates the Dockerfile.local to use go1.18.

This is part of https://github.com/m-lab/dev-tracker/issues/735

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This change is 

[SaiedKazemi]

Dockerfile.local line 1 at r1 (raw file):

FROM golang:1.18-alpine3.14 as ndt-server-build

Can we use golang:1.18 (instead of golang:1.18-alpine3.14) so we don't have to install git, gcc, etc. on the next line?

This is actually what I did for dash, disco, uuid, and uuid-annorator.

Also, per official Go documentation: This variant is highly experimental, and not officially supported by the Go project (see golang/go#19938 for details). Details: https://hub.docker.com/_/golang

[coveralls]

Pull Request Test Coverage Report for Build 1622

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 12 unchanged lines in 5 files lost coverage.
• Overall coverage decreased (-0.5%) to 81.295%

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Files with Coverage Reduction	New Missed Lines	%
ndt5/ndt5.go	1	89.5%
ndt5/c2s/c2s.go	2	73.21%
ndt5/protocol/protocol.go	2	83.86%
ndt5/web100/web100_linux.go	2	94.44%
ndt7/upload/sender/sender.go	5	65.91%
<!--	Total:	12		-->

Totals
Change from base Build 1613:	-0.5%
Covered Lines:	1795
Relevant Lines:	2208

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💛 - Coveralls

[SaiedKazemi]

Dockerfile.local line 2 at r2 (raw file):

FROM golang:1.18 as ndt-server-build
RUN apk add --no-cache git gcc linux-headers musl-dev openssl bash

Since you are using golang:1.18, you shouldn't need this line (and apk doesn't exist).