Add PREFIX to make install

[worxli]

Commit #3bfdc27 broke the Dockerfile. I.e. COPY --from=builder /go/bin/mtail /usr/bin/mtail is trying to copy from a non-existent file.

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[coveralls]

Pull Request Test Coverage Report for Build 2850

• 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 66.341%

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Totals
Change from base Build 2843:	0.0%
Covered Lines:	3597
Relevant Lines:	5422

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💛 - Coveralls

[worxli]

I signed it!

[googlebot]

CLAs look good, thanks!

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[jaqx0r]

Thanks! Docker builds don't get tested by ci yet