Bump actions/cache from 2.1.5 to 2.1.6 (#29)

Bumps [actions/cache](https://github.com/actions/cache) from 2.1.5 to 2.1.6.
- [Release notes](https://github.com/actions/cache/releases)
- [Commits](actions/cache@v2.1.5...v2.1.6)

Signed-off-by: dependabot[bot] <[email protected]>

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

.github/workflows/docker.yml

@@ -26,7 +26,7 @@ jobs:
           components: rustfmt, clippy
 
       - name: Cache cargo registry
-        uses: actions/[email protected].5
+        uses: actions/[email protected].6
         with:
           path: ~/.cargo/registry
           key: docker-registry-${{ hashFiles('**/Cargo.lock') }}
@@ -35,7 +35,7 @@ jobs:
             docker-
 
       - name: Cache cargo index
-        uses: actions/[email protected].5
+        uses: actions/[email protected].6
         with:
           path: ~/.cargo/git
           key: docker-index-${{ hashFiles('**/Cargo.lock') }}
@@ -48,7 +48,7 @@ jobs:
           head -c16 /dev/urandom > src/secret.key
 
       - name: Cache cargo build
-        uses: actions/[email protected].5
+        uses: actions/[email protected].6
         with:
           path: target
           key: docker-build-${{ hashFiles('**/Cargo.lock') }}