frontend: correct docker:cmd directive form

`serve` is treated as a shell form and hence runs:

```
/sbin/tini -- /usr/local/bin/frontend /bin/sh -c serve
```

Which is incorrect. Instead, we want `["serve"]` which is treated as parameters
to `ENTRYPOINT` by Docker, i.e.:

```
/sbin/tini -- /usr/local/bin/frontend serve
```

Author: Stephen Gutekanst

cmd/frontend/main.go

@@ -1,5 +1,5 @@
 //docker:user sourcegraph
-//docker:cmd serve
+//docker:cmd ["serve"]
 //docker:env CONFIGURATION_MODE=server
 //docker:env PUBLIC_REPO_REDIRECTS=true
 

enterprise/cmd/frontend/main.go

@@ -1,5 +1,5 @@
 //docker:user sourcegraph
-//docker:cmd serve
+//docker:cmd ["serve"]
 //docker:env CONFIGURATION_MODE=server
 //docker:env PUBLIC_REPO_REDIRECTS=true