Update Go version in Dockerfile

[cristinaleonr]

A lot the project's dependencies were upgraded when we updated to the latest version of ndt-server #1056.

This has caused the Docker build to fail since it some of the new dependencies require a higher version of Go. https://hub.docker.com/repository/registry-1.docker.io/measurementlab/generate-schema-docs/builds/5dba9aee-3d0c-41b0-90e3-2166e489bb19.

Upgrading the Go/Alpine versions in the Dockerfile to be the same as ndt-server fixes this issue https://github.com/m-lab/ndt-server/pull/339. Tested locally with docker build . -t etl.

This change is 

[coveralls]

Pull Request Test Coverage Report for Build 7207

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 4 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.07%) to 64.421%

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Files with Coverage Reduction	New Missed Lines	%
active/active.go	4	88.54%
<!--	Total:	4		-->

Totals
Change from base Build 7203:	-0.07%
Covered Lines:	3902
Relevant Lines:	6057

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💛 - Coveralls

[cristinaleonr]

Fyi @SaiedKazemi