Use updated docker image (with new rust version)

Author: ALSchwalm

.github/workflows/rust.yml

@@ -6,7 +6,7 @@ jobs:
   fmt:
     runs-on: ubuntu-18.04
     container:
-      image: adamschwalm/hypervisor-build:14
+      image: adamschwalm/hypervisor-build:15
       options: "-u 0:0"
     steps:
     - uses: actions/checkout@v1
@@ -17,7 +17,7 @@ jobs:
     runs-on: ubuntu-18.04
     needs: fmt
     container:
-      image: adamschwalm/hypervisor-build:14
+      image: adamschwalm/hypervisor-build:15
       options: "-u 0:0"
     steps:
     - uses: actions/checkout@v1
@@ -30,7 +30,7 @@ jobs:
     runs-on: ubuntu-18.04
     needs: fmt
     container:
-      image: adamschwalm/hypervisor-build:14
+      image: adamschwalm/hypervisor-build:15
       options: "-u 0:0"
     steps:
     - uses: actions/checkout@v1

Makefile

@@ -1,6 +1,6 @@
 CARGO?=cargo
 BUILD_TYPE?=release
-BUILD_REPO_TAG=14
+BUILD_REPO_TAG=15
 DOCKER_IMAGE=adamschwalm/hypervisor-build:$(BUILD_REPO_TAG)
 
 TEST_IMAGE_REPO=https://github.com/mythril-hypervisor/build