chore/fix: switch release pipeline to goreleaser & ghcr.io (#43)

* chore: add goreleaser config + adapt Dockerfile
* chore: cleanup makefile and deprecated pipeline
* chore: go mod tidy -go=1.19
* feat: add release pipeline

---------

Signed-off-by: Matthias Riegler <me@xvzf.tech>

.goreleaser.yaml

@@ -0,0 +1,35 @@
+project_name: docsonnet
+before:
+  hooks:
+    - go mod tidy
+builds:
+  - binary: docsonnet
+    env:
+      - CGO_ENABLED=0
+    goos:
+      - linux
+      - darwin
+    goarch:
+      - amd64
+      - arm64
+checksum:
+  name_template: 'checksums.txt'
+dockers:
+- image_templates:
+  - "ghcr.io/jsonnet-libs/docsonnet:{{ .Version }}-amd64"
+  use: buildx
+  dockerfile: Dockerfile
+  build_flag_templates:
+  - "--platform=linux/amd64"
+- image_templates:
+  - "ghcr.io/jsonnet-libs/docsonnet:{{ .Version }}-arm64v8"
+  use: buildx
+  goarch: arm64
+  dockerfile: Dockerfile
+  build_flag_templates:
+  - "--platform=linux/arm64/v8"
+docker_manifests:
+- name_template: ghcr.io/jsonnet-libs/docsonnet:{{ .Version }}
+  image_templates:
+  - ghcr.io/jsonnet-libs/docsonnet:{{ .Version }}-amd64
+  - ghcr.io/jsonnet-libs/docsonnet:{{ .Version }}-arm64v8