Use Alpine 3.15 as a base Docker image

build/Dockerfile

@@ -17,7 +17,7 @@
 # IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
 # CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
 
-FROM alpine:3.14.3
+FROM alpine:3.15
 ARG botkube_version="dev"
 LABEL name="botkube" \
     version="${botkube_version}" \