feat: more complete Dockerfile (#243)

Adds several dependencies to the `Dockerfile`, so that Tanka can be
actuall used from there:

- `kubectl`, using the offical binary download
- `jsonnet-bundler`, compiling from source
- `less`, `git` and `diff` using alpine packages

Dockerfile

@@ -1,9 +1,23 @@
-FROM alpine
+# download kubectl
+FROM alpine as kubectl
+RUN apk add --no-cache curl
+RUN export VERSION=$(curl -s https://storage.googleapis.com/kubernetes-release/release/stable.txt) &&\
+    curl -o /usr/local/bin/kubectl -L https://storage.googleapis.com/kubernetes-release/release/${VERSION}/bin/linux/amd64/kubectl &&\
+    chmod +x /usr/local/bin/kubectl
 
-# less with `--RAW-CONTROL-CHARS` is required for tk show/ tk diff
-RUN apk add --no-cache less
+# build jsonnet-bundler
+FROM golang as jb
+WORKDIR /tmp
+RUN git clone https://github.com/jsonnet-bundler/jsonnet-bundler &&\
+    cd jsonnet-bundler &&\
+    make static &&\
+    mv _output/jb /usr/local/bin/jb
 
+# assemble final container
+FROM alpine
+RUN apk add --no-cache coreutils diffutils less git
 COPY tk /usr/local/bin/tk
+COPY --from=kubectl /usr/local/bin/kubectl /usr/local/bin/kubectl
+COPY --from=jb /usr/local/bin/jb /usr/local/bin/jb
+WORKDIR /app
 ENTRYPOINT ["/usr/local/bin/tk"]
-
-