Fix lint Dockerfile

Author: nhooyr

ci/lint/Dockerfile

@@ -1,10 +1,12 @@
-FROM codercom/playcicache
+FROM golang:1.12
 
 LABEL "com.github.actions.name"="lint"
 LABEL "com.github.actions.description"=""
 LABEL "com.github.actions.icon"="code"
 LABEL "com.github.actions.color"="purple"
 
+RUN apt update && apt install -y shellcheck
+
 COPY entrypoint.sh /entrypoint.sh
 
 CMD ["/entrypoint.sh"]