Lower Docker req to 19, make sure we test locally

Ref #15

``` $ docker --version
Docker version 19.03.11, build 42e35e6
```

But abra works. So I think 19 might be sufficient?

Also, wondering if we should add `DOCKER_CONTEXT=default` to that check,
to make sure we're not querying a remote server. (Semi-related question
- should we also check remote Docker versions?)

abra

@@ -479,9 +479,9 @@ require_yq() {
 }
 
 require_docker_version (){
-  major_version=$(docker version --format "{{.Server.Version}}" | cut -d'.' -f1)
+  major_version=$(DOCKER_CONTEXT=default docker version --format "{{.Server.Version}}" | cut -d'.' -f1)
 
-  if [[ "$major_version" -lt 20 ]]; then
+  if [[ "$major_version" -lt 19 ]]; then
     error "This tool requires Docker v20 or greater. Please upgrade your Docker installation"
     exit 1
   fi