travis has been dead for years, have some actions

[mrq1911]

fixes #1.

the repo had no ci at all — deploy-image.sh is still travis-based, so #16, #17 and #18 all merged on nothing but local verification.

• go job: vet, test, build inside golang:1.12, the same image the dockerfile builds with, so ci can't drift from the release build
• docker job: builds the four architectures deploy-image.sh releases (amd64, arm64, armv7, armv6) via --build-arg GOARCH, no emulation since go cross-compiles

verified all four locally and checked the binaries are actually per-arch, not four copies of amd64:

amd64   ELF 64-bit LSB executable, x86-64
arm64   ELF 64-bit LSB executable, ARM aarch64
armv7   ELF 32-bit LSB executable, ARM, EABI5
armv6   ELF 32-bit LSB executable, ARM, EABI5

two things deliberately left out:

• no gofmt gate — handler.go, main.go, router.go, client.go and stats.go all have pre-existing drift, so it would be red on arrival. worth a separate cleanup commit.
• nothing matrixed over docker api version. the issue title says "test matrix", but the tests use httptest fakes rather than a real daemon, so there's nothing for it to exercise yet. would need integration tests against dind first.

the docker job takes ~10min because the dockerfile does COPY . /build/ before go build, so each arch redownloads modules. restructuring it to copy go.mod/go.sum first would fix that, also separate.