[dockerfile] Remove ShellCheck Segmentation fault workaround
[dockerfile] Remove ShellCheck Segmentation fault workaround

The workaround came from this GitHub issue comment:
https://github.com/koalaman/shellcheck/issues/1053#issuecomment-357816927

Looks like the problem doesn't exist any more:

```
.
.
.
Step 14/33 : RUN shellcheck -V
 ---> Running in 4b7a718c1cbf
ShellCheck - shell script analysis tool
version: 0.7.0
license: GNU General Public License, version 3
website: https://www.shellcheck.net
.
.
.
```

The workaround should be removed so that the Dockerfile will be easier
to be understand and read.

ljharb/nvm,creationix/nvm,creationix/nvm,PeterDaveHello/nvm,PeterDaveHello/nvm,ljharb/nvm