locale-gen command seems missing since uncertain point for a while:
```
Step 15/33 : RUN locale-gen en_US.UTF-8
---> Running in fae073c89c01
/bin/bash: locale-gen: command not found
The command '/bin/bash -o pipefail -c locale-gen en_US.UTF-8' returned
a non-zero code: 127
```
This patch should fix the Docker image build
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.
Use pre-built static release binary instead of building ShellCheck from
the source code, this change would save disk space, time, and much
computing resources.