website: Fix building instructions

- Reordered the dependency instructions to have 20.04 dependencies
listed first.
- Fixed incorrect docker image label.

Change-Id: I5336c65770632ae8bc8230a7baf59b3ee2d6e169
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5-website/+/60012
Maintainer: Bobby Bruce <bbruce@ucdavis.edu>
Reviewed-by: Bobby Bruce <bbruce@ucdavis.edu>
Tested-by: Bobby Bruce <bbruce@ucdavis.edu>
diff --git a/_pages/documentation/general_docs/building/index.md b/_pages/documentation/general_docs/building/index.md
index fc490d4..713f788 100644
--- a/_pages/documentation/general_docs/building/index.md
+++ b/_pages/documentation/general_docs/building/index.md
@@ -51,17 +51,6 @@
 libraries. It is a necessary dependency if you wish to use the SystemC
 implementation.
 
-### Setup on Ubuntu 18.04 (gem5 >= v21.0)
-
-If compiling gem5 on Ubuntu 18.04, or related Linux distributions, you may
-install all these dependencies using APT:
-
-```
-sudo apt install build-essential git m4 scons zlib1g zlib1g-dev \
-    libprotobuf-dev protobuf-compiler libprotoc-dev libgoogle-perftools-dev \
-    python3-dev python libboost-all-dev pkg-config
-```
-
 ### Setup on Ubuntu 20.04 (gem5 >= v21.0)
 
 If compiling gem5 on Ubuntu 20.04, or related Linux distributions, you may
@@ -73,6 +62,17 @@
     python3-dev python-is-python3 libboost-all-dev pkg-config
 ```
 
+### Setup on Ubuntu 18.04 (gem5 >= v21.0)
+
+If compiling gem5 on Ubuntu 18.04, or related Linux distributions, you may
+install all these dependencies using APT:
+
+```
+sudo apt install build-essential git m4 scons zlib1g zlib1g-dev \
+    libprotobuf-dev protobuf-compiler libprotoc-dev libgoogle-perftools-dev \
+    python3-dev python libboost-all-dev pkg-config
+```
+
 ### Docker
 
 For users struggling to setup an environment to build and run gem5, we provide
@@ -84,7 +84,7 @@
 https://gem5.googlesource.com/public/gem5/+/refs/heads/stable/util/dockerfiles/ubuntu-20.04_all-dependencies/Dockerfile)).
 
 Ubuntu 20.04 with minimum dependencies:
-[gcr.io/gem5-test/ubuntu-20.04_all-dependencies:v21-2](
+[gcr.io/gem5-test/ubuntu-20.04_min-dependencies:v21-2](
 https://gcr.io/gem5-test/ubuntu-20.04_min-dependencies:v21-2) ([source Dockerfile](
 https://gem5.googlesource.com/public/gem5/+/refs/heads/stable/util/dockerfiles/ubuntu-20.04_min-dependencies/Dockerfile)).