Update docker_install.md

[cosperypf]

"cd $WORKSPACE" should be "cd $WORKSPACE/aimet", otherwise the CMakelist file will not be found in the next "cmake .." step

[cosperypf]

if only "cd $WORKSPACE" and "mkdir build && cd build", we will get the error as bellow when we enter "cmake .."
"CMake Error: The source directory "/data" does not appear to contain CMakeLists.txt."
The right thing to do is "cd $WORKSPACE/aimet".

[quic-akhobare]

Thanks for pointing this out @cosperypf

[quic-akhobare]

Could you please sign the commit, like so

[quic-mangal]

@cosperypf, can you sign the commit as pointed out by Abhi.