In the previous commits the ARCH
env variable was introduced to the Dockerfile
and build_container.sh scripts. It was meant to
help with configuration of linux headers for
musl-tools, but did not work for some reason.
Considering the build_container.sh was already
using `uname -m` calls to get the current arch
this commit also uses it, so there is no need
for ARCH anymore.

Signed-off-by: Egor Lazarchuk <[email protected]>

Removed && from the script because they
were causing the script build to succeed
even if errors were occurring.

Signed-off-by: Egor Lazarchuk <[email protected]>

There are several places that need current arch
of the system to install/configure the container.
This commit replaces separate calls of $(uname -m)
with one global ARCH variable.

Signed-off-by: Egor Lazarchuk <[email protected]>