Thank you @mr_cal and @jamesh! That was indeed the issue. In the previous build (on a 18.04 machine) the …/…/…/ scheme would correctly move to the wanted folder as the build directory was held in that respective part’s directory. So to fix the issue I had to create a directory on the container, mount the directory to it, and then for that part do the correct …/… etc. scheme to get to it. But now I have a different error. 
If you are able to help with this issue:
Building wickr
+ snapcraftctl build
/opt/qt51512/bin/qmake 'LIBS+="-L/root/stage/lib"' 'LIBS+="-L/root/stage/usr/lib"' 'LIBS+="-L/root/stage/lib/x86_64-linux-gnu"' 'LIBS+="-L/root/stage/usr/lib/x86_64-linux-gnu"' 'INCLUDEPATH+="/root/stage/usr/include"' 'INCLUDEPATH+="/root/stage/usr/include/x86_64-linux-gnu"' CONFIG+=aws_wickr_branding CONFIG+=release CONFIG+=use_wickr_npl CONFIG+=qzxing_multimedia CONFIG+=qtquickcompiler CONFIG+=wickr_alpha CONFIG+=use_autoloader CONFIG+=snapcraft /root/parts/wickr/src/../../../../WickrDesktopApp/desktop.pro
/opt/qt51512/bin/qmake: /lib/x86_64-linux-gnu/libm.so.6: version `GLIBC_2.29' not found (required by /opt/qt51512/bin/qmake)
/opt/qt51512/bin/qmake: /lib/x86_64-linux-gnu/libc.so.6: version `GLIBC_2.28' not found (required by /opt/qt51512/bin/qmake)
Failed to run '/opt/qt51512/bin/qmake \'LIBS+="-L/root/stage/lib"\' \'LIBS+="-L/root/stage/usr/lib"\' \'LIBS+="-L/root/stage/lib/x86_64-linux-gnu"\' \'LIBS+="-L/root/stage/usr/lib/x86_64-linux-gnu"\' \'INCLUDEPATH+="/root/stage/usr/include"\' \'INCLUDEPATH+="/root/stage/usr/include/x86_64-linux-gnu"\' CONFIG+=aws_wickr_branding CONFIG+=release CONFIG+=use_wickr_npl CONFIG+=qzxing_multimedia CONFIG+=qtquickcompiler CONFIG+=wickr_alpha CONFIG+=use_autoloader CONFIG+=snapcraft /root/parts/wickr/src/../../../../WickrDesktopApp/desktop.pro' for 'wickr': Exited with code 1.
Verify that the part is using the correct parameters and try again.
Im thinking it could possibly be because my version of qt51512 is oriented for ubuntu 20.04 (current machine is 20.04) but the the container might be 18.04. or at least when I run " cat /etc/lsb-release" in the terminal on the container it says:
DISTRIB_ID=Ubuntu
DISTRIB_RELEASE=18.04
DISTRIB_CODENAME=bionic
DISTRIB_DESCRIPTION="Ubuntu 18.04.6 LTS"
On local with cat /etc/lsb-release it gives:
DISTRIB_ID=Ubuntu
DISTRIB_RELEASE=20.04
DISTRIB_CODENAME=focal
DISTRIB_DESCRIPTION="Ubuntu 20.04.6 LTS"
So is there a way for the container to be 20.04? I ran ldd --version on local and the container.
For local I get:
ldd (Ubuntu GLIBC 2.31-0ubuntu9.9) 2.31
Copyright (C) 2020 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
Written by Roland McGrath and Ulrich Drepper.
On the container I get:
ldd (Ubuntu GLIBC 2.27-3ubuntu1.6) 2.27
Copyright (C) 2018 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
Written by Roland McGrath and Ulrich Drepper.
Unsure though.