Simplify Docker instruction compilation

Summary:
After adding a number of new instructions, it seemed there was a lot of
redundant implementation between concrete types. This refactor uses just
one concrete unexported type and a few options for determining the
output format instead of one struct type per possible Docker
instruction.

The exported `docker.Instruction` interface and signature of
`docker.NewInstruction` is unchanged with this refactor. Unit tests for
Docker instruction compilation were simplified to not make type
assertions but were otherwise left to ensure no regression.

Depends on D984

Test Plan: Run `go test ./...`.

Reviewers: thcipriani, demon, hashar, mmodell, #release-engineering-team

Reviewed By: thcipriani, #release-engineering-team

Tags: #release-engineering-team

Differential Revision: https://phabricator.wikimedia.org/D990

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