Verify doctl sha256 in build.

Dockerfile

@@ -12,13 +12,16 @@ LABEL "com.github.actions.icon"="droplet"
 LABEL "com.github.actions.color"="blue"
 
 ENV DOCTL_VERSION=1.18.0
+ENV DOCTL_SHA256=f556cd224d5da65edbb4f50a927bfbf01892f9488782f5cf146b4d28d6fe0b22
 
 RUN apk add --no-cache curl
 
 RUN mkdir /lib64 && ln -s /lib/libc.musl-x86_64.so.1 /lib64/ld-linux-x86-64.so.2
 
 RUN curl -L https://github.com/digitalocean/doctl/releases/download/v${DOCTL_VERSION}/doctl-${DOCTL_VERSION}-linux-amd64.tar.gz  | tar xz -C /usr/local/bin/
 
+RUN /bin/echo "${DOCTL_SHA256}  /usr/local/bin/doctl" | sha256sum -c -
+
 COPY entrypoint.sh /
 ENTRYPOINT ["/entrypoint.sh"]
 CMD [ "help" ]