Fix 1 Security, 3 Maintainability issues in multiple files

[pavelskipenes]

CodeFactor found multiple issues:

Possible OS Command Injection (CWE-78)

• src\exec.cpp:21

COPY to a relative destination without WORKDIR set.

• Dockerfile:7

Always tag the version of an image explicitly

• Dockerfile:1

Use the --no-cache switch to avoid the need to use --update and remove /var/cache/apk/* when done installing packages

• Dockerfile:5